I think his point is that with the right training and experience and put in the same job there are plenty of people just as smart, diligent and organized as Cook. I suspect that is true statement, but the assumptions aren't really fair. Where could those 100,000 people get the training and experience that Cook has? How could they prove to the Apple Board that they have the same or better skills than Cook? Since they can't do either, they really aren't competition for Cook. But I think the general point is true on some sort of theoretical level. The same could not be said, for example about an elite basketball player like Lebron James. We know that there aren't thousands of people out there with his size and speed, so we know he cannot easily be replaced. But CEOs, yes it seems their pay is a little high in comparison to what they seem to bring to the table.
But who knows, perhaps Cook has some special set of skills that are less common than 0.1%. I think Jobs had skills like that.