iMessage (public) keys are "backed up" to IDS directory services. This is how Apple devices do key-exchange with one another. If there is no man-in-the-middle mitigation for IDS, then all the Chinese government needs to do is return spoofed keys for anyone you're communicating with. Remember, when you send messages to other people, you encrypt with THEIR key, not yours (on device). Your key is used for signatures and decrypting messages sent to you by them. THEIR Key comes from the cloud, and thus subject to attack if key exchange wasn't secure.