Earlier quoted context omitted.
iCloud isn't some separate entity from iMessage. It's all Apple. And you have no option to use a different cloud backup provider. You can decide not to give your keys to Apple, but you can't decide for all your friends to not give their keys to Apple, and the result is the same: Apple can read your messages. And the marketing is so misleading that hardly anyone knows that Apple can read most iMessages.
Sorry, let's be explicit here, as you seem intent on muddying the issue. Where, other than the endpoints, is the message decrypted when people use iMessage? Your succinct answer to that will clear this up for everyone.
"If you have iCloud Backup turned on, your backup includes a copy of the key protecting your Messages. This ensures you can recover your Messages if you lose access to iCloud Keychain and your trusted devices."