You're right, it is a big assumption, but it's not unfounded. I've worked at F50 companies and founded my own startup. It's a lot easier to get two really great designers from the far right side of the bell curve when you're a small startup. As an organization reaches even 1000 people, you're now starting to draw from the middle of the bell curve. In fact, you have to. If you try to hire only from the far right of the bell curve for all positions, you end up with a lot of egos that will clash. In the best case, you hire leaders from the far right of the bell curve and followers from the middle. But at some point those mediocre followers start asking for promotions and your hotshot leader leaves for greener pastures. Controlling for that in your hiring practices at a large organization is virtually impossible, particularly if you have standard (middle of the bell curve) HR people. BTW, this is exactly why startups out-execute large companies every day of the week. A small startup can carefully control its hiring, select from the right side of the bell curve, and avoid all the large company HR crap. But as soon as it starts to scale, the bell curve becomes a looming threat.