You might be interpreting the "claim" more rigidly than is warranted in a popular publication. Personally, I think children are born potentially smart, but intelligence must be developed constantly in my experience. I have found that many people limit their own intelligence by not stressing themselves, and I suspect the same is true for outside influences. There is a reason people associate bookishness with intelligence and I don't think it's a one-way relationship. I have known a few people whom I believe are not potentially smart, but not that many.