Apple also claims that iMessage is end to end encrypted. Their privacy stance is 99% posturing.
Also I thought you could trust iMessage if, unlike everyone, you disabled iCloud backup (and, unlike everyone, so did the recipient). Perhaps a way for the feds to be able to pin dumb criminals while giving investigative journalists & dissidents a way to stay safer.
https://support.apple.com/en-us/102651#advanced
You do have to be sure to not enable web access via icloud.com
Much easier to use a 3rd party app like Signal.
https://support.apple.com/en-us/118246
https://support.apple.com/en-us/118247
> An unrecognized new device was added to that person’s Apple Account. This alert might mean that the person you are messaging has an issue with one of their devices, or that a sophisticated attacker might be attempting to eavesdrop on the conversation.
My understanding is that iMessage implements PFS. To get around PFS and access older messages, one needs to get their hand on a backup, which needs fully enrolling a device, not just messaging key exchange hackery.
And as far as trusting Apple with key exchange, well, if you're running their OS and hardware, I suppose that trust of key exchange is the least of your concern (or part of the whole deal anyway depending on how you look at it)