I think they deliberately underplayed their role in this. Especially with Asian parents who think such nurturing is part of the "norm". I wouldn't be surprised that they spent TONs of time tutoring him when he was young -- and when he was more or less self-bootstrapped they don't need to spend too much time.

But I could be wrong. He is definitely a genius so maybe he did grasp the ideas rather early, like from 3 or 4.