[mindslight]

> Only the sole keeper of the code can modify the code, it's proprietary software

LOL!

> Further the code is signed by author's private key

This is the crux - if Apple is in a privileged position to defeat security measures and you're analyzing security in terms of Apple/USG, this counts as a backdoor. It doesn't provide full access, but it does undermine purported security properties of the system.

It's quite possible to implement a system with similar properties that doesn't give Apple such a privilege. It sounds like they didn't.