As long as I found the Nazi disturbing, the "denialism" in Tianmen square massacre or gulag or Ussr stalin purge or Uyghurs genocide from the far-left ideology is as much disturbing as the same from the far-right one. Just because this instance seems leftish and anti-Nazi, it doesn't leverage itself to a better position.
They probably won’t ever stay fully out, since anyone can make an instance, just up to mods and individuals to block them and not federate with obvious natzi communities.