My own (probably unqualified) 2 cents: ActivityPub protocol is open-source, so we can't stop them from using it and asking to federate with other instances. And blocking Meta is, by design, something done on an instance by instance basis. Even if Threads does become the largest AP protocol-based platform - how is that really different from now? If they try to Extend, then they just break connection to the rest of the Fediverse and it's back to being the silo everyone wants to escape.