Yeah I just don’t know how the relay could relay DMs without exposing the npubs of both parties.
Discussion
They would have to handshake and exchange a new (derived) npub that will be used for communication. Nobody would know who those npubs belonged to, so non-parties would see a single message from A to B after which they could communicate via private npubs. Like I tell you: DM me at npubXYZ, then you create npubABC and DM me over there.
Again I'm just spitballing, but seems plausible to me at the moment.