From what I found, a lot of the people who join Threads, wouldn't have even looked into joining the Fediverse in the first place. If anything, Threads is bringing the concept of the Fediverse to more people and some of those people may find that Mastodon and the rest of the Fediverse could be what they prefer, especially if you can still talk with your family and friends on here.
Threads are stuck behind dealing with their investors as we can push the Fediverse with new updates all the time