Any child who read maths textbooks with enthusiasm for 3-4 hours a day for years could in theory at least get close to doing what he did, but what kid had that level of motivation?
There is no way this is true. I've met and worked with enough people to know that not everyone has the same mental ability. There are some exceptionally sharp people and many dim witted ones too.
I don’t say everyone has the same mental ability. But I stand by my point. Those people you mentioned might be dimwitted in part _ because_ their lack of enthusiasm for learning is low, so they didn’t do it. I don’t care how smart you are, effort matters.