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.