Modified from an earlier answer of mine and since the OP started first with the anachronism of environmentalist/environmentalism, for clarity's sake, so will I.

There's a lot to unpack in these questions and the charge that the Nazis were green is one that has been and still is instrumentalized by opponents of the environmental movement. Snyder's argument about anxieties is part of a larger thesis in Black Earth, and I'll leave it to /u/commiespaceinvader 's answer on Snyder. I will say that Snyder often falls into a myopic love with his grand sweeping paradigms that explain complicated historical events and processes. The problem is that even though his historical models are often quite elegant, they do not explain as much as he purports they do and they are nowhere near as novel as he claims. Both Black Earth and Bloodlands often bang on open doors and even when historians agree with Snyder, this quality is often quite grating.

Environmentalism within the Third Reich had a very strained and complicated relationship with the state and its dominant ideology of National Socialism. German environmental thinkers often had to adjust and shift their thinking to fit this new climate. For many, this was an easy task since the German environmentalism of the 1920s and 30s shared a number of congruencies with völkisch nationalism that stressed that Germans were deeply connected to certain types of landscapes and climates such as the forest or bucolic countryside. But the transition was seldom easy and in many cases this was environmentalism in a National Socialist key. Moreover, the Third Reich found some environmental thinkers of greater utility than others. Agronomists that could make the land bloom or zoologists seeking to find the genetic key to breed cows into their original auroch form found state patronage much easier than other Germans whose research and interests did not so closely align with the principles of National Socialism. This dynamic of selectively engaging environmentalist thought and self-coordination of many Germans environmentalists would become the dominant leitmotifs of environmentalism within the Third Reich.

One of the clearer examples of this process was the Reichsnaturschutzgesetz (Reich Nature Protection Law, or RNG). The RNG was arguably the most comprehensive law for the protection of the environment in the world when it was enacted in 1935. Yet the same state that enacted the RNG, which stipulated that any public works project needed to consult local conservationists, it also advanced a ruthless exploitation of natural resources in the name of economic autarky. In numerous cases, the needs of the war economy triumphed over the legal principles behind the RNG. Many German environmentalists played a much less prominent role in the direction of state policy as it evolved in the late 1930s. They could gain traction when the needs of remilitarization were not apparent, but getting the state to apply the RNG was much less likely to if environmental protection came at the expense of industry. For example, Frank Uekötter’s monograph on Nazi-era conservation, The Green and the Brown, notes that environmental protectionists in Baden were able to prevent the construction of a hydroelectric plant in the Wutach Gorge during wartime through their exploitation of local patronage networks to ensure that the state enforced the RNG. By contrast, the earlier efforts of the Heimat activist Ludwig Finckh to close the neighboring basalt quarry at Hohenstoffeln Mountain hinged on the quarry no longer being economically feasible.

Similar contradictions were present in the Third Reich's management of peasant agriculture and seed distribution. One of the foremost proponents of the NSDAP's program of "Blood and Soil" was Richard Walther Darré who became Agriculture Minister in 1933 and promoted a centralized bureaucracy Reichsnährstand (RNS) to promote a peasant agriculture. Darré's various writing stressed that the German peasant ideally lived in a symbiotic harmony with the land, and the role of National Socialism was to encourage this symmetry. The RNS promoted certain seed strains and coordinated with various agronomist research in various institutes, whom the RNS often reminded that they had to benefit small-holder farms. The RNS's policies created an initial period of prosperity for Germany's peasants, largely because of protectionism and subsidies, but the good years did not last beyond 1936. The pressures of autarky pushed the RNS to advocate greater yields and seed strains for peasant farmers. This centralization implicit in the RNS meant that peasantry found themselves locked out from a system that was ostensibly for their own benefit. The turn to more commercial and high-yield farming also made a mockery of Darré's claim to promote a völkisch agriculture that eschewed modernity and returned the peasantry to its primeval roots.

Environmentalist ideas often became associated with Volkisch sentiments at points within the Third Reich that the latter often outstripped the former. German writers on the environment conceptualized the environment in racialized terms during the Third Reich. A healthy forest or land was a sign of an Aryan land. Hans Klose, who wrote a majority of the RNG, Klose, who wrote a majority of the RNG’s text, in a December 1939 speech claimed that although Germans have always possessed a strong affinity to nature, it was only after 1933 that the Germans possessed a government that sided with nature. Klose encouraged his audience to fully comply and educate the public on rationale behind the RNG and conservation in general. Similarly, a 1942 editorial from Gartenbau im Reich that reflected a radicalization of environment was Max K. Schwarz’s 1942 “Zeitgemäße Gedanken über Garten- und Landschaftsgestaltung.” Schwarz’s perspective was that landscaping, whether individualized gardens or the planning of cities, had to reflect a racialized nature. “The landscape,” he asserted, “is not determined by the determination of a single thought, but rather by the entire Volk. Only the Volk itself can comprehensively design the countryside.” According to Schwarz, the dynamic character of National Socialism had made it possible for human beings to more completely understand themselves and their role in nature. He then developed a theory of total nature which is not to be enjoyed just for its aesthetic value but for the insights they can give into the Volksgemeinshaft. For example, he instructed his readers that “plants are should not only be appreciated go for passive viewing, but appreciated more so for their organic and influential connection with the body and their own environment.” This vision of nature was only to be cherished and enjoyed through the unique character of the German race. This implicit embrace of a racialized conversation efforts was even more noticeable in Walter Schoenichen’s editorial “Naturschutzgebiete der neuen Reichsgaue.” Appearing in the December 1941 issue popular middle-class magazine Westermann’s Monatshefte, Schoenichen described, in vivid language, the unspoiled paradise in the newly-absorbed territories in Poland and the protectorate of Bohemia-Moravia. According to Schoenichen, the experience of a raw unblemished nature needed to be protected by a large national park system found in other nations. Such a park system would be “an uplifting thought that [a national park] would be dedicated to the highest and proudest summits of the Greater German Reich which is devoted to the spiritual edification of the German Volk and its reverence for nature.”

Of course, when dealing with the issue of pollution and environmental degradation, German writers would invoke the familiar spectres of Poles and Jews. SS-affiliate authors for General Plan Ost would claim that soil erosion and depletion of the environment were typical behaviors of inferior races and the SS-led new order would erase this damage through the Grünaufbau (greening) of Polish and Soviet lands. “Hag und Heimat” by Heinrich Friederich Wiepking-Jürgensmann, Heinrich Himmler’s special representative for the Reich Commissariat for the Consolidation of German Nationhood (RKfDV) and a prominent prewar landscape architect, drew a very tight connection between this new Heimat and the Grünaufbau. Wiepking-Jürgensmann explained to the public in the article “Hag und Heimat” the spirit of the special relationship between the Heimat and nature developed under SS patronage. “Hag und Heimat,” written in a clear and colloquial style, contended that this symbiosis was fully congruent with the racial principles that the SS also protects in the Greater German Reich. For example, he wrote that “the landscape is an unmistakable mirror of the Volk and their entire being; in the neglected landscape the greatest robber is only the most efficient inmate of the land.” The double-meaning of this passage indicated that not only has the SS liberated the Volkdeutsche in the Eastern territories, but its previous non-German governments mismanaged the land so much as to corrupt its inhabitants.

backdrop: I find Snyder's work often monocausal and even simplistic. (Haven't read Black Earth yet, but I definitely thought Bloodlands was a lot of old stuff--included discredited theories of Totalitarianism--repackaged with a simple-sounding sound-bite...)

This is itself a simplistic (and time-constrained) first stab at your complicated question, but here goes:

Landscape preservationism and environmentalism both have very long historical lineages in Germany (both having been more or less 'invented' there), and they thus both played into Nazism in a similar way to other ideologies (Lutheranism, Socialism, colonialism, etc.) that fed into, factored into, and were used by Nazism: namely, in complicated ways.

First, the majority of Nazi leaders (and likely, of Nazi members) were far more worked up about a few key issues (namely, anti-Versailles Treaty, anti-semitism, anti-socialism, and eventually, war) and "green" thinking was nowhere near a primary concern of theirs. It barely factored into the thinking of any of the top Nazis (with the possible exception of Goering--the self-styled "Reich Master of the Hunt" and "Master of the German Forests"... though he seems to me to have been more of an outdoor enthusiast and quasi-conservationalist than any sort of true environmentalist.)

As to your question about being an ancillary concern (German Romanticism, aesthetic appreciation of/by "the German peoples", Blood & Soil, etc.), "ancillary" is a good way to phrase it. "Nazi thought" was a chaotic miasma--almost an open field for anyone to leap in and promote their own agenda.

There were many adherents of National Socialism before and after 1933 who were also advocates of or influenced by landscape preservationism and even environmentalism, and so 'brought' these ideas into the party (and were sort of "green evangelists" within the NSDAP). After 1933, they could "use" Nazi rhetoric and Nazi politics to advance their ideas (when given an appropriately-Aryan gloss.) The language of "Heimat" (homeland) was very useful in this context, as it was easy to use Romantic notions of "Heimat" to rhetorically and ideologically bolster landscape preservationism with the racial thinking that saturated Nazism.

So preservationists saw opportunity in Nazism... in part, because of top Nazi's claims that they would rein in capitalism in favor of something more "German." (They did not.)

And of course, there were the military-economic concerns of autarky, which could also carry a conservationist gloss...

That's all I have time to type, now, but here are some sources:

There is a strong essay collection How Green Were the Nazis? that came out a decade ago. (can't remember the editors, though)

Three other great books on this topic (all academic histories, though all are quite readable):

Thomas Lekan Imagining the Nation in Nature: Landscape Preservation and German Identity, 1885-1945

Frank Uekötter, The Green and the Brown: A History of Conservation in Nazi Germany

David Blackbourn, The Conquest of Nature: Water, Landscape, and the Making of Modern Germany

EDIT: to come back to your point about contemporary partisan uses of this to smear environmental protection policies, you are certainly correct. The same is done with "Socialism"--even though Nazis were hardly socialists. An on-point retort might be: the Nazis were "environmentalists" in the same way that they were "Christians"...that is, opportunistically, tactically and brutally, and in a way that exaclty inverted the true meaning of these ideas they were both borrowing and abusing.