> If I am an app developer how does it tell me that your OS has not been tempered with or actually that my app has not been tempered with?

This is not your business to verify and control what can run on my phone. I can do it with my smart card, which securely stores cryptographic keys.

> And as a user, you do not have your iCloud or iTunes account used for device attestation.

It does not matter. An account is necessary to make the phone usable at all. The attestation is useless on a phone that can't install apps.