Please correct me if I’m misunderstanding. Key distribution was necessary because in other implementations (except NIP-04), users don’t post the message using their actual npub. Clients had to generate an alternate key pair to encrypt the message, so it looked like the message was posted by some random npub. Therefore it wasn’t compatible with private relays because the relay didn’t have that npub on file. 

I’m hoping that this NIP solves that problem so that encrypted DMs can be sent to private relays which use an allow-list of npubs that have permission to post.

If that’s not the case for this NIP, my next question is: how do private relays support it?