I think there's a difference between being able to backup a file (like I can with keepass password database) to any device,format or location I choose, vs backup my iPhone to Icloud,because for all intents and purposes that's just secondary memory/storage for my one device.

My wife recognizes the scream from the home office when I try to move ANYTHING from my iPhone to anything else in the world. I have no doubt that backing up the pass keys from iPhone / ICloud to a thumb drive or my windows pc would lead to even more primeval shrieks.

So I feel those pass keys are pretty stuck to that device in practical ways that matter. Let me know if I'm wrong!