I hear the point you're making and I respect the angle, its fair-enough, but ...
The trouble with venturing into what-if territory is the same applies to you...
What if the disk you took out was subjected to an evil-maid attack ?
What if the crypto implementation used on the disk you took out was poor ?
What if someone had infiltrated your OS already and been quietly exfiltrating your data over the years ?
The trouble with IT security is you have you trust someone and something because even with open-source, you're never going to sit and read the code (of the program AND its dependency tree), and even with open-hardware you still need to trust all those parts you bought that were made in China unless you're planning to open your own chip-fab and motherboard plant ?
Its the same with Let's Encrypt certs, every man and his dog are happy to use them these days. But there's still a lot of underlying trust going on there, no ?
So all things considered, if you did a risk-assessment, being able to trust Apple ? Most people would say that's a reasonable assumption ?