What's the difference? We're substituting having superior intellect with having the willpower to work hard. It seems weird to say that at first. But the more I think about it, the more I think there must be a reason why some people do persevere enough to become "genius" level at something, and some people never do.

Personally, I think work ethic, along with intellect and charisma, is something you are partially born with. It's just silly to say that genetics play no factor: do those with severe mental disabilities just not work hard enough? Think about it as your genes defining a range: you can be between the 30th and 75th percentile of human prowess. Where you fall in that range is up to you.