ATproto does support storing data elsewhere. That’s what a PDS does. I’m not sure what you mean by an offline-first system in this context though or why it’s required for decentralization. Could you elaborate?
Yes, and it only supports storing one's data in one place. If we use local-first as our measuring stick for a decentralized system then ATProto fails since we can't store the msgs anywhere -- such as on the nodes of my friends. Prior art such as secure-scuttlebot had this figured out, and thus no one ever complained about ssb being centralized.