He isn't saying "There exist exactly 20 people out of 60k who should not go to college and I have oracular ability for identifying them." He is saying "It is arrogant to assume that, out of 60,000 people attending college, it is impossible to find so many as 20 of them who would be better served by another alternative." Presumably, he thinks the actual numbers is substantially higher than 20, but he doesn't have to get locked in to the extremist position of saying "60,000 of 60,000 people should not go to college" to believe that. It could plausibly be that 99% of people are served perfectly adequately by college and identifying only 20 of the remaining 600 isn't really rocket science compared to the actual rocket science that other Paypal alums are doing.
