Why not both? Replacing their existing Twitter profile with Mastodon doesn’t remove the possibility to integrate AP into their platform. It probably makes it more likely.
The WaPo bans showed that social media should be controlled in-house (or at least appear to be, like email.) Medium is just a bit ahead of the curve. Expect others to follow.
In the worst case scenario where discussion is mostly held on a few instances owned by vampires, we’re still better off because federation is a helpful feature.
https://fedibb.ml