I'm not sure what they mean by "demobilized" exactly, but I'll try to give a general look at the state of the Romanian left in that period to try and fill you in. One thing that's worth noting in general is that the socialist movement (and the left in general) was quite small in Romania compared to the movements in most Western and Central European countries, which makes sense given that Romania was still a largely agrarian society that was still in the early stages of industrialization during the mid-to-late 19th century, when socialist movements were developing in Europe.

There wasn't a proper socialist party in Romania until 1893, when the Social Democratic Workers' Party (Partidul Social-Democrat al Muncitorilor din România) was founded. The most important Marxist theoretician in Romania during this time was Constantin Dobrogeanu-Gherea, a prominent literary critic who had been born in Ukraine and had fled political persecution by the Okhrana due to his socialist activities in Russia; along with Constantin Stere, he was a key figure in the Poporanist movement, which was a nationalist and populist movement akin to the Narodnik movement in Russia. The socialist movement in Romania at that time was primarily focused in Bucharest, which was the most industrialized city in Romania and also had a large student population, where socialist ideas like those of Dobrogeanu-Gherea, which were more radical than those of Stere, found some currency.

The PSDMR was a marginal political faction, with a peak membership of about 6,000 people in the late 1890s. However, in 1899, a large portion of the party's members (those who were influenced by Stere's version of Poporanism) defected to the Liberal Party, which was one of the two main political parties in prewar Romania, while Dobrogeanu-Gherea's faction remained with the PSDMR. This split basically killed the PSDMR, which dissolved in 1905.

The schism in the PSDMR is important because it presents one of the central tensions in both Romanian leftist politics and prewar Romanian politics more generally: the divide between those who embraced the new, Western-oriented political and social structures that had formed following Romania's independence and those who held to more traditional Romanian values. Although both Stere and Dobrogeanu-Gherea's ideas were populist and inclined toward embracing popular tradition, Stere's faction was more willing to work within the existing political system, which led its defection to the Liberal Party. However, it should be noted that these ideological divides basically took place in a vacuum, given the lack of a large industrial working class; the prewar socialist movement in Romania was largely an intellectual movement dominated by figures like Dobrogeanu-Gherea and Stere and populated by university-educated urbanites rather than a truly "proletarian" movement. Given your interest in anarchism, it's also worth noting that the more social-democratic ideals of Stere and Dobrogeanu-Gherea played an important role in marginalizing the influence of anarchism within the Romanian left during this period, although it was revitalized somewhat after the collapse of the PSDMR.

This dissonance between socialist rhetoric and the decidedly un-proletarian composition of the socialist parties in Romania continued until World War II. It's also important to note that like Dobrogeanu-Gherea, many of the prominent figures in the Romanian socialist movement prior to World War II (like the Bulgarian-born Christian Rakovsky and the Hungarian-born Elek Köblös) were immigrants to Romania from other parts of Eastern Europe, which led to a further disconnect between the nationalist aspirations of the native population and the internationalist ideas of the largely non-Romanian socialists, further undermining the socialists' ability to build a broad base of support. This situation changed somewhat after World War I, when the territorial expansion of Romania brought in parts of the former Russian Empire (Bessarabia and Bukovina) with stronger socialist movements and more revolutionary political orientations, although the socialists largely remained marginal until the Soviet occupation of Romania in 1944.

Anyway, after the dissolution of the PSDMR, the more radical wing of the party, which had refused to join the Liberal Party, took a few years to re-establish themselves, eventually founding a new socialist party, the Social Democratic Party of Romania (Partidul Social-Democrat din România), not to be confused with the present-day PSD, which isn't really connected to the older PSD. The PSD was also unsuccessful as an electoral force, since they still lacked a natural working class base of support and were walled off by the Romanian electoral system.

At the beginning of World War I, the PSD took a pacifist stance, opposing Romania's entry to the war on either side, and participating in the Zimmerwald Conference of pacifist socialist movements in Switzerland in 1915. The PSD's pacifism probably induced the government to crack down on its activities, and it was banned outright after Romania entered the war in August 1916. Many of its leaders were either arrested or conscripted into the Romanian armed forces, effectively disbanding the PSD. Bucharest, which was the PSD's traditional base of power, was occupied by the Germans at the end of 1916, and the Russian Revolution prompted a further crackdown on the socialist movement in the unoccupied regions of eastern Romania which bordered Russia. Most of the remaining leaders of the socialist movement were imprisoned by 1917.

After the war ended, the surviving former members of the PSD joined with the leaders of the socialist movements in the newly-gained territories of Romania (Transylvania, Bessarabia, Bukovina, Banat, and southern Dobruja) to form the new Socialist Party of Romania (Partidul Socialist din România), which would eventually become the Romanian Communist Party (Partidul Comunist Român) in 1921; a splinter group from the PSR, who opposed affiliation with the Communist International, re-adopted the Social Democratic Party moniker for their new party in 1927. As I mentioned before, even after the consolidation of the socialist movements from across Romania, socialism remained a marginal political force in Romania throughout the interwar period (a problem only exacerbated by the execution of many of its leaders, like Rakovsky, Köblös, and Dobrogeanu-Gherea's son, Alexandru, during the Great Purge), and the PCR only gained political power after the Soviet occupation of Romania led to the imposition of a communist-dominated government in the late 1940s.

As I mentioned at the beginning of this answer, I'm not sure exactly what they meant by "demobilize", but the tl;dr version is that the socialist movement wasn't that strong to begin with and the banning of the main socialist party, along with the arrest or conscription of its leaders, effectively killed it off until after the war, and the lack of a genuine proletarian base of support for the party prevented spontaneous action in response to the repression of the PSD.

Sources:

Keith Hitchins, Rumania 1866-1947 (Oxford UP, 1994)

Vladimir Tismaneanu, Stalinism for All Seasons: A Political History of Romanian Communism (U of California Press, 2003)