Yes they are certainly more dissimilar, or the argument can at least be made in terms of a viewpoint of modernity.

The Dutch speak Dutch, whereas Germans, Austrians, and Swiss speak German. It’s important to note French, Italian, and Romansh are also spoken in Switzerland, and that all of these places have their unique identities.

However, the Dutch distinguish themselves in a few ways. They have a separate history as an independent republic, the most “free” nation on earth at the time of its existence, notable for establishing “freedom of thought.” During the Dutch Golden Age in the late-16th and 17th centuries, the Dutch Republic dominated world trade, conquering a vast colonial empire and operating the largest fleet of merchantmen of any nation. Though Germany has ample coastline in the north, their traditional focus on maritime activity has generally been far less. Austria and Switzerland are landlocked.

During this Golden Age, the County of Holland was the wealthiest and most urbanized region in the world. Artists flourished under this regime, including painters such as Rembrandt, Johannes Vermeer and many others. So did scientists, such as Hugo Grotius and Antonie van Leeuwenhoek. At this time, the other countries you mention were under sway by the awkward bounds of the Holy Roman Empire.

Ethnically it’s hard to say, though one can assert a few things. Austria is the birthplace of the Celts. The area roughly comprising Switzerland was part of the Roman Empire, while Germany largely was not. Austria has ties to Hungary, via the Austria-Hungarian Empire - a world power up from the mid 19th century until WWI - and that historical Hungarian influence is certainly non-Germanic in character in a number of ways, though how real that is is hard to say. Honestly, I wouldn’t particularly look at it as an ethnic thing as much as a linguistic and cultural one.

Grouping them all together in general is a bit silly, but I think the point is the linguistic dominance of the German language as well as the contrast of the cultural achievements of the Dutch fairly early in the modern period of European history, leading to a distinct identity.

You have to start with "ethnicity" and the evolution of it's meaning.

First map is a "reprint" of an german ethnic study during end of 19 century, and it follows "self-identification" reporting.

Austria had never recovered after Bismark war, and the cultural Austrian "elite" which was speaking "same" high german started to feel "German" because it was cooler thing to do. (high means hills and mountains, i.e. german spoken in mountain/hill areas). I am not sure the Swiss german speakers had agreed to be pointed blue on this map.

Second map had to include some border of the Denmark actually. The dialect map is continuous and the grammar/pronunciation/ word use changes were rather gradual (up to the end of 1960, so it is good documented).

Regional cultural dependencies don't follow countries borders either.

North west Germany and the east Netherlands were very close, i.e. the percentage of people having relatives on both sides of the border was very high, and the people had no issues to move and work in another country. This family relation collapsed during Hitler occupation and the forceful attempt of the germanization which led to the dutch aversion to anything german (which persists even today).

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The root of the problem with questions like these is that it implicitly rests on the assumption that Germans (as a people, as an ethnicity) predate the Dutch; whereas the revers is true!

What we call Germans today (and retroactively - and often anachronistically) project back into history) are essentially a residual category. The Germans are (and I do not mean this disparagingly at all) are those West Germanic tribes/ groups which did not form a new and sharply defined ethnicity or nation during the Middle Ages or Early Modern Period.

The English did, the Dutch did and - in many respects - the Swiss did so as well; but the Germans did not. Only during the 18th century did the idea of a well defined German nation truly gather momentum.

After 1871, Germany found itself as one of Europe's largest and most powerful nations and actively promoted a kind of romantic nationalism to strengthen its newly found nationhood. Within that ideology, Germany and the Germans were no longer on the fringes of other European cultures; but formed it center. Suddenly, peoples like the Swiss, the Dutch, but also the English, were thought of as having sprung from German roots. After all, didn't the English come to England from Saxony?

As far as this pangermanism went, only the Austrians really played along with it. Which resulted, among other things, in the sometimes still vague ethnic/cultural relation between Germans and Austrians. The extent of Swiss "German-ness" never really extended beyond the use of standard German; whereas the English and Dutch pretty much ignored it completely or only used it in a kind of meta (Germanic rather than German) context.