They don't need an identifier, they have both user's IPs/session IDs and channel IDs available. Date/time of messages is also precise. Connecting into multiple friends with the same IP reveals more about you. If the server decides to track (which a court order might require them to) the metadata is probably useful. The beauty of the GiftWrap design is that people can setup their own Inbox relays at will. The package only arrives in the receipient's server, not in the sender's if the person doesnt want to (save a copy locally only). That with random date/times makes it harder for relays to track. Then of course, clients can always use a separate Tor circuit to make sure relays don't receive anything but the message in each connection.

How is it different from setting up your own SMP receiving relay?

Correct me if I am wrong but the SMP contains both user messages. In Nostr each user relay has the peer user messages. Relay can't count the full conversation.

Had to read the above a few times as I'm not very technical. Are you suggesting nostr will be able to provide better privacy compared to SimpleX? How can Nostr hide the metadata? And last question, is something like 0xChat potentially the way?

Yep, we hope to do better than SimpleX. The implementation 0xChat and Amethyst have is the same and already this new model :)

Brilliant, is there anywhere I can find this explained? Thank you

I've looked into it a bit more and can see myself taking advantage of your nip but no common relays or relay implementations really support it right now so will just wait and observe.