Famously, Bill Gates would review both code and low-level technical architecture decisions of flagship products.

This shouldn’t be the exception, it should be the norm.

I can always tell which company sells products the CEO doesn’t use and has likely never even seen.

I think so too, I'm impressed that he wants to personally look at code at all. His ability to evaluate it deeply is not relevant, one thing is for sure: he can ask deeper questions than if he had no code to talk about. That alone is a big bonus when you need to do an evaluation of a person, having something concrete to talk about.