They can do that without federating. In the first 7 hours of Threads being open, they got 10 million users. There's nothing additional they can do to "extinguish" the fediverse simply by being inside it.
Any fediverse users who prefer Threads are going to go there anyway - remember, you have to create a new account on each server already! And anyone who detests Meta is going to stay in the fediverse regardless. They're here now, when the fediverse is minuscule. Nothing Meta can do is going to make the fediverse smaller.