Isn't it also the case that each instance gets updates from all others? So in this case, lemmy.world's access to beehaw communities might lack user@beehaw.org activity and beehaw's access lacks user@lemmy.world activity?

I've seen conflicting info on this. It seems to be the case that beehaw's communities are hosted by beehaw as the "true version" which is synced with everyone else. In this regard, lemmyworld is blocked as beehaw will not send the "true version" of the community to lemmyworld. As for whether lemmyworld can get the "true version" from another instance? I don't know. Lemmyworld does have the beehaw content before they were blocked, which they can still see and engage with. However those interactions are not sent to beehaw, which is where other instances are syncing with. Whether or not lemmyworld can indirectly participate in beehaw communities via other instances is unclear to me but I'm guessing the answer is no.

But I also believe lemmy.world can see old beehaw stuff from before the defed, so that could play a role.

This is correct. Lemmyworld still has a copy of all the beehaw stuff from before the block/defederation. However, I believe it's the case that if lemmyworld users try to comment on it, those comments will not be sent to us here on kbin (since we get our copy of beehaw communities from beehaw).

notably: lemmyworld users can see beehaw user posts on kbin magazines.