It's not a cover, it's an explanation :)

Caplan shouldn't be surprised at the reception of his work, because people have this prior.

Of course as you say high priests of rationality examine their own biases as they reach for the book, and by the time they flip it to read the recommendations on the back they are already empty vessels for unadulterated pure non-chill-filtered high-octane data and nothing else. But most people working in public policy are just lowly humans. :/