I've been asked before about how Nostr differs from ActivityPub based social media like Mastodon a few times. My answer is both are decentralized but each is decentralized with a different topology.

Both Mastodon and Nostr are decentralized social media or communication platforms, however Mastodon is federated while Nostr is distributed. Mastodon is decentralized by means of having no central server by making each participant of the federated network their own centre, while Nostr is decentralized by means of none of them being a centre. Nostr relays cannot control or enforce their own rules in the same level as a Mastodon instance can.

You could say the same about an instant messenger, some are decentralized by means of having no central server, just multiple centers that can communicate with one another (like XMPP) or by letting you pick the relay servers anyone can host (like SimpleX) or by not having any servers at all and communicating between each other peer to peer (like Briar).

Not all decentralized messengers are the same, they are decentralized in different manners.