Is this why .world preemptively defederated from hexbear?

It's generally the reverse. In a Western-dominated space, Communism goes against that grain. It's impossible for a Communist in the west to actually insulate themselves from the dominant narratives online. On the flip side, it's extremely easy to never hear from Communists, as they only tend to occupy their own spaces, all you need to do is defederate or ban. That's why Lemmy.world has more of a claim to being an echo chamber than Lemmy.ml, Lemmy.world never even gives Hexbear or Grad users a chance to comment to begin with.