Unlike P2P, messenges are sent over easily replacable relay servers so both parties don't have to be online at the same time to exchange messenges.
SimpleX is special because it is the only messenger that has no permanent user identifiers whatsoever. Not even a randomly generated string of characters and numbers. This implies a LOT more privacy and security, especially for metadata-protection.
The server of Signal for example can't read your messages, but they know who sent how many messages at what time to which people. They know your entire graph of social connections. SimpleX does not.
Here's a comparison with other messengers: https://github.com/simplex-chat/simplex-chat/blob/stable/docs/SIMPLEX.md#comparison-with-other-protocols