Like the rest of us, Healy doesn't like the idea of being tested at his job. If we are doing a good job, we worry the test won't show it. If we are not, we worry it will. So we invent elaborate reasons why we can't be tested, reasons that rarely impress the guy who must pay the bills and is not interested in taking our word that we deserve a raise.
In the case of colleges, we are that guy. Healy wants us to spend four years of our lives and five figures of our limited income at his or another institution of higher education. We have important alternatives for both our time and money. He can hardly expect us to shell out without having some way of testing the results he says are so wonderful.
Granted, proper testing is difficult and can easily distort your view by ignoring other factors. But difficult and impossible are quite different and we routinely test everything, a matter of necessity. The salesman's word that this is a great product just can't be accepted.
Healy shows this himself when he claims that a college education means one ". . . will never again mistake ignorance for knowledge." He then provides two bad examples not to be copied, both of whom were college educated. (Indeed, despite Healy's claim, most college grads assume that since they know a lot about something, they know a lot about everything. Of course, this is also true of most non-college grads too, but Healy is trying to identify benefits, not areas where college doesn't make us greatly worse.)