Apparently, lemmy.ml already blocked Threads before Threads started using ActivityPub.
Personally speaking, I don't like it if administrators start deciding for all users on their server what they can and cannot see (although they obviously have the right to do so).
If I want to follow a community or user in Threads, why should an administrator of a Lemmy server be able to prevent me from doing that? It also sounds rather silly, as it's terribly easy to create multiple accounts on various Lemmy servers and read Threads posts that way.