Threads hasn't even implemented ActivityPub yet, and there's no guarantee it will do so in the future. Or even that it will implement it in a way that links with lemmy.
I personally haven't seen a post from lemmy.world admins pledging to block threads. Even if you do, take it with a grain of salt.
I've personally blocked communities and users that talk about reddit/threads all the time. It's cool that they have a space to do it, I'm just not super into those topics right now and can always unblock them later.