For instance, you identify "collective ownership of the means of production" as central to the idea of socialism, and the USSR as a society with collective ownership of the means of production. There's a tricky elision there, though, in that it identifies "state ownership" with "collective ownership." Chomsky and other anarchists raise hell at that idea because to them, the State is not an instrument of collective decision making.

For more context: anarchists played a major role in revolutionary socialist Russia, with the popular party of the peasantry (the Socialist Revolutionaries) being in some ways one of the more anarchist political parties in history (supporting, for instance, land and industry collectivization instead of land and industry nationalization). They were quite popular, having over twice the number of representatives as the Bolsheviks, the second largest party. They quickly came to oppose the Bolsheviks and were among Lenin's first victims. This is is in addition to a hardcore anarchist movement also popular in Russia at the time (and also heavily victimized by the Bolsheviks).

The point of all that? They all considered themselves socialist and disagreed that Bolshevism would lead to socialism, disagreeing strongly enough that it often led to their deaths. Chomsky isn't pulling the "the USSR wasn't socialist" out of a hat, but is bringing up an ideological point that's existed since even before the USSR formally existed. And to an anarchist, the idea that the USSR was socialism is as alien to them as the idea that the USSR was capitalist is to you. (The fact that the USSR called itself socialist they would argue was just a propagandist slogan, just as a Tea Partyer might consider Obama calling himself a capitalist as meaningless political rhetoric to take advantage of the less critically minded.)