The difference is that each client can create as many rotating keys to receive the message as needed, including a new one per message if it is really necessary. These are created by the client, the server doesnt know about them.

But the main component is that you can encrypt the already encrypted message to a separate pubkey as many times as you need. You can basically create an onion route using multiple queue IDs where every server only knows where the which node is coming from and going to but not the author and the receiver. Messages can use complely separate routes, making it impossible to track a conversation sequence. 

On top of that, dates and times are random. Servers don't know which messages are new and which are not. Many payloads might contain the same messages, adding noise to the network. 

All of that is done with a single message type: A GiftWrap event.