And can you prove that Apple cannot access the key? Oh, yes because Apple marketing says they cannot.No. Not even Apple can access the key. That's the whole point. If you can prove otherwise, it would be headline news, and you could probably make a career out of it. And bear in mind that very clever people are testing the security of Apple's phones, for good or evil, every day.
Sadly, common sense is rarely common. Or sensible.