There was a lot of stuff going on between 1956 and the 1962. People who became "Socialist Realists" back in the Stalin era remained realists (see Konstantin Yuon in 1956 (1912, 1922, 1935), Deyneka in 1959 (1920s,1937), Alexander Plastov in 1961 (1928, 1937).

The new generation (born in the 10s and 20s) was modernist but not necessarily impressionist. In the aforementioned 1962 Manege exposition (that made Khrushchev go berzerk) you had mostly expressionists: Vera Preobrozhenskaya, Lucian Gribkov or Vladislav Zubaryov.

Of course you also had different things in that era in the different parts of the USSR (Ülo Sooster is what? A surrealist? And what is Pyotr Belenok?). The renowned Soviet abstract artist from Saint Petersburg was Alexander Arefyev and indeed he sometimes was an impressionist, so it may be that the collection you have seen was drawn from what's called the "Arefyev Circle" in St.Pete's.

Literature: in "the Magical Chorus" by Solomon Volkov there is a chapter on Western collections of "Soviet Underground Art" like the one in Pittburgh. Perhaps it will explain why the one you have had mostly impressionists.