The only thing I have a hard time trusting Intel with is the way they have handled questions around their Management Engine. I think they have done a poor job convincing us that it's safe or at least benign.
It will be interesting to see if Apple includes a secretive management engine. If they do, then the speculation that it's required by one or more governments will be dialed up to 11.
With Apple's documented interest in owner's control of their devices and delivering the required documentation for it, they won't tell us what is included in the chips and what's not.
Fortunately, Apple regularly publishes their iOS Security Guide as criddel mentioned below and there is a vibrant Apple chip reverse engineering community that does microscope analysis of Apple's chips [1].
I always see this document linked to on HN even though it's a (relatively) high level description of what is happening on the device.
Besides, there is absolutely no guarantee that Apple isn't just being "semi-open" with their security mechanisms. In other words, who is to say that there are no redundant, undocumented mechanisms in place?
But I will concede that Apple is ahead of everyone else in this regard. It's just that I think Apple being ahead doesn't mean that the information they're providing is enough.
You're right that a whitepaper isn't enough to be really sure. That takes disassembly of binaries and then you are still stuck with judging the trustworthiness of the hardware and I'm not sure how you solve that problem.
Apple is probably going to produce a similar document that details at a high level how the security works. If they follow the example that they set with iOS devices, what they won't do is provide the source code for these components.
I mean, of course, it's still their right to release or keep any source code they produce. We shouldn't be expecting them to release the source code, but even a whitepaper is better than what Intel is doing.
It will be interesting to see if Apple includes a secretive management engine. If they do, then the speculation that it's required by one or more governments will be dialed up to 11.