If you read his last two novels he was clearly well-informed about mathematics. However I have to assume that McCarthy didn't actually master all the material in the math books mentioned here, I think the reporter may be a little too credulous about that. I suspect he had the very common experience of buying a yellow book and being defeated in the first couple chapters.
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.