Private routing is on by default now. As of v6 (or maybe v5.8, I don't remember). You're not wrong about channel IDs, but it's a different ID per message queue. With 2 queues, receiving and sending, and each member's pair using different queues, they have very little info to start tracking users, and it would be very hard to identify anyone from that, on a server that has a lot of messages going on.