correct, beehaw and sh.itjust.works users can both read and post to communities on lemmy.ml, but they can’t see eachother’s posts or comments (because the beehaw admins defederated from sh.itjust.works).

btw, I (as an admin at lemmy.ml) am deleting this post from asklemmy because it is a lemmy support question.

This was discussed in the early days of federation and the consensus was that a block should be a hard-block as most likely the offending instance isn’t contributing in a good way on 3rd party communities either.

However it looks like the allow-list is less strict and lets comments from non-allowed instances through if they are on 3rd party communities. But I have not confirmed this myself.