The "success" in this study is that regular kids in a regular class end with 10% of kids testing for gifted, and regular kids in a "gifted" class show 15-20%. That simply is not that impressive or surprising.
The problem is that good instructors and resources are limited. And while those things would help anyone, you maximize their value by giving them to the kids on the gifted track. We, as a country, would be worse off if we didn't do this because we would be letting the truly talented go to waste in the name of "fairness."