> However I have to assume that McCarthy didn't actually master all the material in the math books mentioned here

Why? And what exactly would mastery look like? Regardless, McCarthy didn't make his mark as a mathematician so his private ability to understand doesn't matter. Why take the opportunity to make a negative assumption and diminish the possibility that he had mastered an understanding in his private life? What does this accomplish? Seems like the only thing it could possibly do is to try and make you and me feel better about our own inadequacies, without proof.