Tuxer said "keys," not "your decryption keys." Apple distributes the public keys that each party encrypts their message with, and they route the encrypted messages through their servers. They can trivially eavesdrop on conversations by simply providing a key from a key pair they generate to a participant and reencrypting messages using the other parties' public keys after deciphering the messages.

https://threatpost.com/apple-imessage-open-to-man-in-the-mid...