I agree with you

Though it could be simpler if it was something like a magnet on the lid that activates a magnetic switch on the bottom part (and it would be harder to have a false negative result). But Apple is going to Apple

Yes, it could be done with a Hall effect sensor or something like they used to. The cool thing about this approach is they actually use a different angle to turn the screen off as you close the lid than they do for turning it on when you open the lid, to create a better experience. Since it is a security feature, then the "open" vs "closed" state should use the same source of truth. So it's a trade-off of complexity and experience.