If they are so sure what's necessary for communism why did every state they support not wither away into socialism but collapse into disaster capitalism? Did they just not shoot enough people?
"The tradition of all dead generations weighs like a nightmare on the brains of the living." - Karl Marx
All the 20th century communists outside of communist countries that they idolize didn't believe those things really happened in socialist countries. When it became well known that they did, most people abandoned the ideology. 40 years ago these people would be telling you this stuff didn't happen, it's a peculiar and ahistorical type of internet brain damage that people are now willing to fully embrace gulags and mass murder as goods of the system.
The people who know this stuff happened and instead of abandoning their support for totalitarian states, try to justify the actions of Lenin, Stalin, Mao, Pol Pot, etc are not functionally any different from fascists.
Antifa must stand against all genocide fetishizers or what's the point?