[sneak]

> That's an optional feature (that I do not use),

It doesn't matter. Everyone you chat with uses it because it's on by default, so all of the iMessages you send and receive are backed up in effectively plaintext to Apple (who turns them over to third parties).

> But none of that is a backdoor in the encryption of iMessage and you do the whole space a real disservice by conflating them.

Unencrypted (or encrypted to the ZK middle service, in this case Apple, being the operator of both iMessage and iCloud Backup) key escrow of end-device secret key material in a system that is advertised as end-to-end encrypted is indeed a backdoor in the end-to-end encryption of that system, as now the secret keys don't exist just on the endpoints - the transit service in the middle has a copy of them, allowing message decryption on a non-endpoint as they transit the middle service.

That is definitionally not end-to-end encrypted. It's end-to-middle-and-end encrypted if the middle device has a usable copy the endpoint secret keys, which Apple does.