Interesting follow up to this thread. I was looking over the AT Protocol FAQs about why they don't use Activity Pub.
It basically boils down two things. The first is perceived security problems of migration from one server to another. The logic sounds very rooted in a block-chain conception of security. The second are aesthetics. They don't like how usernames work in Activity Pub and problems around search. Basically, it felt like grasping for straws.