Point well taken, but I'm still not convinced that the variability you describe couldn't be accounted for by things like differences in diet (which can greatly affect cognitive ability), or differing circumstances at home (ie. abusive homes, or parents who are neglective vs caring and nurturing) and/or outside the home (like various forms of stress when kids live in dangerous neighborhoods where their peers and adults get assaulted or murdered on a regular basis), parents who push their kids to achieve vs ones who don't care how or what their kids do, etc...
Picking out the effects of nature vs nurture in a random chess playing kid is not easy, and I wouldn't be so quick to attribute to innate talent that which could be explained by environment.
What is clear, however, and something which you did not seem to deny in your own response, is that playing a lot of chess when you're young is critical to overall chess performance. Without that even "intelligent" and very capable adults who started playing too late are just not going to catch up to capable kids who played a lot of chess early.