Except that ActivityPub and Nostr’s moderation functionality is basically identical. Relay/instance operators can block users, filter content, set their own moderation policies, and defederate from other instances with weak moderation policies. The difference is that if your instance admin blocks you from following somebody you want to on AP, you need to make a new account at a new instance and check that account seperately. If your instance admin does that in nostr, it’s just a matter of adding another relay to your list and now you can keep following/being followed by/DMing that person. It’s the best of both worlds: relays can set their own moderation policies and cultivate a certain vibe, users and their identities are not locked in to the moderation policies of the instance(s) they are connected to.
Protocol wise, you can absolutely use a portable identity with ActivityPub. Every user has a key pair that us used to sign and verify their posts, and there’s no reason why you wouldn’t be able to use the same key for multiple servers. Nobody actually implements a scheme like that but you could use keys instead of ActivityPub usernames to label accounts, if you wanted to. You could even use multiple servers the same way nostr uses multiple relays!
You can do this, but your account is still tied to the instance. If somebody sends a DM or tweet to [email protected] but lemmy.ml no longer exists, all the public keys in the world don’t solve that problem. In nostr, tweets and DMs are directed at a key, not at a user at a particular relay.