I think the point is, that even if they are intelligent and capable and definitely contributed a lot to their initial codebases ( I don't know why you singled out Gates, but he most certainly did as well), all of these companies grew to the point where almost any code change requires a bunch of expertise with technologies that they probably have not been exposed to.

I don't know if they wouldn't get hired, I mean it's not like those companies never hire straight out of college, they obviously do, so expertise is not an absolute requirement. But they probably couldn't be hired as drop-in experienced SE's. Half their tech stacks are on technology that didn't even exist when they last touched code.

All of these CEO's are famous for their excellent early talent acquisition of software engineers. Microsoft and Apple for leveraging those engineers to initiate a new digital world, and Facebook and Google for attracting (and motivating) the worlds best to achieve global domination.