No, iMessage for example generates a unique key pair per device and submits your public key to the key pool for your account. Devices can then sync old history using each other's keys and new messages will be encrypted for all devices.
The problem is that the key pool is in Apple's hands. You cannot guarantee that they don't go and add a key do the pool and get all your messages anyways.
The problem is that the key pool is in Apple's hands. You cannot guarantee that they don't go and add a key do the pool and get all your messages anyways.