The key here is that it is D.C. schools. In any other city you would see a difference.
They've done similar studies in Milwaukee, New York City, and I believe Cleveland, with rather mixed results.
I don't think any of the studies has shown a definite advantage for vouchers, but I do know that in Milwaukee, for instance, there was a lot of fraud in some of the private schools, which might have skewed the results.
On the other hand, if vouchers were used nationwide, I think there would continue to be lots of fraud in some of the private schools, so maybe that's not such a skew after all....