You missed the hedge phrase—"pretty much". Obviously there isn't such a thing as a guarantee for security, but there are counterexamples for most OSs. Do you have one for the current iOS version? I haven't seen someone jailbreak their phone since iOS 5, I think, though people did do it since then.

What's the point of your critique when you can apply it to literally any piece of software? Even SEL4 relies on a correct processor, and yours has flaws.