Says so in the article. Chess has a unified global ranking scheme so you can objectively measure your success in chess.
The real tragedy is that these children grew up under Communism; there's another, even better, unified objective global ranking scheme he could have used if he raised them in a free country.
http://lesswrong.com/lw/jw5/how_much_wealth_is_produced_by_h...
Using a financial metric as opposed to a chess metric is an internal/external validity tradeoff that would support the claim that the parenting techniques used are useful in real-world scenarios. I think this is better because as many others have pointed out, how good you are at chess is usually not a factor in how successful a life you lead, whereas in the West, your net worth is basically a direct measurement of success.
Personally I believe that as far as financial success is concerned, how well you handle transactions is just as important as net worth (so it's best to have a lot of money and handle it really well), but that's impossible to quantify of course.