Hard to separate nature from nurture when you school your kids on your own area of expertise. Had he tried to make them into expert musicians or downhill skiers, his experiment might have more weight.
The experiment has been tried in downhill skiing too, I can think of several competitors at the Sochi Olympics who were coached by their (not expert skier) fathers.
But isn't the idea behind the 10,000 hour idea that you need constant high quality feedback and training?
Nope; just that you've been at it, studying and trying. The (Gladwell) examples are of folks in groups that learned together.
You would think he would have adopted, if he was serious about 'made, not born'.
I believe he wanted to try it later in his life (with adoptees) but his wife talked him out of it..
Would be difficult to school them in any meaningful way if he was not already proficient. Teaching is hard, particularly if you are trying to do something different to the standard (& therefore supported) model.
How about a child adopted at a young age. Trying to teach the kids something you are not is a different proposition altogether.