I read a post by the Beehaw admins a couple weeks ago saying they were talking to the lemmy.world admin about resolving the issues that caused them to defederate, so it’s possible that they were no longer defederated when the post you found was made. My understanding is that automatic updates only happen when users on one instance are subscribed to the community on the other instance, so refederation might not be obvious. I expect they would have cut the cord again over yesterday’s security breach, though.

That’s pure speculation on my part, though, and quite possibly it was some kind of bug. But I am not particularly tech-savvy, so I tend to wonder about non-technical causes.