Probably more taking a shot at mastodon and blue-sky. Blue-sky still hasn’t actually federated with the rest of the fediverse so I doubt whether threads will actually do what they are saying. maybe they are just using it as marketing. They wouldn’t make money from ads if people just used a mastodon instance to follow people on threads.
Personally I think they’d have to charge monthly for their curating algorithms. maybe one day there will be loads of small instances that each have their own bespoke curating systems that you can choose from.
The point of the fediverse in a broad sense is decentralisation. By having bookwyrm as an alternative everything isn’t linked to goodreads. If they ever decide to make it paid only or the company goes down, all that stuff is gone. On the fediverse the posts would be split and hosted across multiple independent communities. This make it more robust Imo