It’s nothing to do with that, they made the AT protocol because is structured in a way that ensures that bluesky (the company) will always control the network. They wouldn’t be able to keep control with ActivityPub, it’s the same reason Threads will never implement it.
All the stuff about account portability is a distraction. Think about it: where would one move an account to anyway? Another BS node? Why? Unlike Mastodon instances there is no functional difference.