Federating doesn't prevent that either, but at least you won't be rewarding them for it by engaging with them. If Meta wants to sink ActivityPub (or rather, subsume it), it will, and no actions we can take will prevent that, bar forking the standard in some way.
In fact, not federating with Threads is the only potential way to ensure that our instances don't become reliant on functionality that Threads adds, even if we can't save the ActivityPub standard itself.