I can't see a large corporation wanting anything to do with the fediverse, they want their own playing field they can fully control to their liking.
Edit: just saw that their app is ActivityPub based. Sooo I guess they are joining the fediverse? Really not happy about this, the last thing we need here is some soulless corporation infecting our growing community.
They want to do the same thing Google did with Jabber/XMPP. Microsoft's: Embrace, Extend, Extinguish.
Google with their Google Talk did exactly that. They introduced their own instant messaging. Mamy XMPP users (including myself) convinced friends to join Google Talk. Once it picked up they first broke communication (it only worked one way, you could see GT people online, you could see them messaging you, but your messages never arrived) eventually they defederated completly. I still feel being used.
Facebook knows that it is tough to bootstrap community when there are no users in it, and they want to do exact same thing.
I think it will be both. Accounts created with Threads will have their own walled garden but will have the option of interacting with other fediverse communities.
There is a very real possibility that it will be designed in such a way that it's initially federated, but will slowly get to a point where they can see mastodon posts but mastodon can't see their posts. (Adding non-activitypub features and standards) All the benefit of having an already active userbase with none of the work for organic growth, and none of the downsides of competition in the long term.