Ed Witten was very good at math when he was young. He scored exceptionally well in the American Mathematics Competition. He didn't pick physics up out of nowhere, he already had a quantitively talented mind.
He was almost implying that Witten came out of nowhere with very little mathematical background and became a physics god. He implied this by saying that Witten studied history and linguistics in college, but he failed to mention his previously strong background in mathematics during his childhood. That last part is the key signal to how Witten could have even possibly become a successful physicist, although of course it is not a sufficient condition.
I can't find it today, but I remember reading the AMC results with my own eyes many years ago. I believe he was one of the best scorers on either the AMC 8 or the AMC 10 in his state.