Sure, but they still have to abide by that instance’s guidelines. And as far as I can tell, it’s easy to subscribe to an external community without ever seeing those guidelines.

I agree fully with this. Centralizing all of the major communities with Beeyah is silly. And I wish the moderation rules were at the community level instead of instance level, but I understand that’s a limitation of the ActivityPub protocol (as far as I can tell).