Few claims in the social sciences are more fully settled. I don't think you could find a researcher in the world at a major university making that claim that randomly selected children could be reliably turned into world-class mathematicians with enough training.

> How else would you explain the Polgar sisters?

Genetics is the obvious explanation. The father was clearly very intelligent.

Also to clarify: I agree that training and effort can have large effects, and that focusing on them is a good strategy. Over-believing in them is probably a good bias, even. But the idea that everyone is more or less the same except for effort is ridiculous.