[hashbig]

This is both true and false. Apple stores keys on the device so they can't read your old messages, but say they want to start reading messages of a particular user, they can simply issue a new key and store it on the device and the server and start decrypting the new messages using it.

This is why WhatsApp for example notifies users when the key of the recipient changes, and they give you a way of verifying that the both keys at both ends are identical.