Nope, it's all backdoored with backups.
Here's the simple setting you need to change on your iPhone...
www.forbes.com
And before you say turn off icloud backups, we've already seen testimony that apple can remotely turn backups on and trigger a backup. They do it all the time for active investigations.
Really, they HAVE to do it that way because people would probably be annoyed if they got a new device and couldn't get their old messages.
Edit; and if they did actually make it secure Apple would be blocked in places like China and Iran etc. Just like Signal is blocked there.