Users on lemmy.world still have a "shadow" version of the beehaw communities on lemmy.world that would normally be synced but isn't anymore. Users on lemmy.world can post to that community and it can be seen by other users there but that post doesn't go back to beehaw or anyone still federated with them.
It means that beehaw can't "hear" anything coming from lemmy.world.
When you write a comment, it gets saved to lemmy.world (or whichever home instance you're currently using). The lemmy.world instance then broadcasts your comment out to the Fediverse. Normally, this causes other instances to see your comment and save a copy locally, which is how it's possible to see your comment on other instances. Because Beehaw ignores lemmy.world, their instance ignores the broadcast containing your comment. It's never saved to Beehaw which stops the users logged in over there from ever having a chance to see it.
In addition to ignoring lemmy.world, Beehaw is also excluding lemmy.world from it's own broadcasts. So, if someone goes and creates a post on Beehaw, the lemmy.world instance will never get told about it thus stopping those logged into lemmy.world from ever knowing it existed. You simply can't interact with something that doesn't exist, even though lemmy.world technically isn't trying to stop you.
What you're seeing here is the overlap between these two effects. The lemmy.world instance knows of old posts that exist on Beehaw, because Beehaw was still talking to lemmy.world back then. Then you as a user on lemmy.world can see the post and write a comment on it, since your comment lives on lemmy.world. Finally, lemmy.world tries to tell Beehaw about your comment and gets ignored, preventing anyone on Beehaw from ever knowing it existed.
What I don't understand is that I've seen posts in Beehaw communities by Beehaw accounts. Surely that shouldn't be possible with defederation? Is there some delayed output that lemmy.world has access to but they can't see our input?
The way Beehaw defederated is that it simply refuses any updates issued from lemmy.world and sh.itjust.works. Updates from Beehaw are still pushed out to multiple sources that lemmy.world and sh.itjust.works can read from.
Some people prefer heavy moderation. That's the beautiful thing about the fediverse, you can choose the kind of moderation you want to deal with (on your home instance at least).