The AT protocol and activitypub are completely different protocols. Bluesky the “distributed” Twitter replacement uses AT, but nobody else does.

The benefit of activitypub is that it is completely decentralized. You don’t need to ask anyone’s permission to start up an activitypub project or server other than maybe a domain registrar.

The benefit of AT is you can maintain a single unified identity across different servers, since there’s basically one source of login credentialing.

At least that’s as far as I understand it, I could be wrong.