It is very different than manually moving instances in the Fediverse, in my understanding. For one, it's totally transparent to the people that are following. All of your data comes with you. You don't lose your posts, your followers don't lose following you. I know Mastodon has at least recently gained account portability to some degree, but https://docs.joinmastodon.org/user/moving/ has some serious drawbacks that aren't present in the AT model.
Activitypub is the federation equivalent of an at-home 3d printer. It’s easy enough to understand how it works and do stuff with it. And very hackable. Bluesky is trying to be more like Firefox: it’s much more complex and built by experts to be well optimised for the problem it’s solving. But it’s much harder for lay people to understand how it works internally.
Maybe it sounds good on paper but in practice it amounts to a nothingburger and is essentially the same as manually moving instances in the fediverse.