I’m not 100% certain yet how federation works with lemmy, but I’m assuming it works similar to how it works on Mastodon and Calckey, only with group actors.
In that case, servers don’t share all of their content with each other wholesale. Someone on your instance would need to already be subscribed to a community on another server in order for it to receive content from it, or to even know it exists.
In order for people to see content from actors (ie people or groups) that aren’t currently being followed by someone on their server, actors they do follow need to boost that unshared content. And I’m not sure how that works on Lemmy.
To subscribe to communities that aren’t currently being shared with your server, go to their home server, find the community there, get the community’s URL, and then enter that into the search bar on your instance.
lemmy.ml is also run by communists, as are quite a few other instances, I imagine. No one’s really saying that communists should be blocked.
But lemmygrad is specifically a vanguard edgelord site. Even those of us who are communists don’t necessarily want to deal witih /c/GenZedong.
Plus, I left the instance I host for me and my friends open to lemmygrad, and I had people from the server create accounts and just bulk subscribe to communities there.
It’s not the communism people are blocking it for.