Those people who knew him seems a good guide. In that video, he's talking about accounting math. Actually, I assumed "math whiz" meant proficiency in proving th

Those people who knew him seems a good guide. In that video, he's talking about accounting math. Actually, I assumed "math whiz" meant proficiency in proving theorems, but the ordinary meaning of being good at arithmetic is reasonable, and probably what's meant here. Although he's a maths geek in that sense, and he also went to Harvard, I think combing those two as "math geek from Harvard" suggests the theorem proving meaning. To clarify: that wikipedia article says he graduated magna cum laude. I think to be a maths whiz, you need to be really good at it.