@hongminhee activitypub has a top-down feel, abstracting "social web" concepts and indeed sticking (mostly) within the Web conventions. It is not very prescriptive about implementations.
In comparison ATproto is more of a bottom-up design, in a sense the bluesky code is the documentation of the protocol. Important design choices target a certain type of "scaling".
Its quite fascinating and revealing that such different approaches, help create the same "twitter" microblogging illusion.