The issue they stated wasn’t with other federated communities, but with the users from those federated instances spamming their communities.

Yes, I understand that, but this seems to be effectively the same. Why not leave the decision to the individual users?

Beehaw has a strict account policy and only want users they’ve personally vetted commenting and posting in their communities.

Well, the fediverse kind of seems to be the wrong choice for them, then. It lives from the federation. If you want to be isolated it’s just a plain old forum.