In the Activity Pub model, the server and the client aren’t separate. It’s essentially a bunch of small twitters which all talk to each other. It’s the Federated model.

A bunch of smaller digital nation states speaking the same protocol.

Your identity is tied to whatever server you join. When you leave that server, that identity, and all connections are lost.

The nostr model separates these things.

Identity is tied to a cryptographic key which you own.

Relays(servers) pass requests from one person to another. One key to another.

Clients exist separately from servers(relays).they provide an individual who has a key, a way to publish to relays with that key, and request events from other keys(people)

In the Fediverse, if the server you were a member of blocked the posts of one person, or a whole other server, you could no longer see or interact with them unless you left and joined another server.

In nostr, if some person ran a relay who decided they were going to not relay notes from someone, you could just add another relay to your client to see their notes. The relay operator was able to act on their (censorious) conviction, and you are still able to route around it without losing your social graph.