Isn't anyone at least a bit curious how an all black team would have performed? If the all black team performs as badly as the all white team, it would only bolster the argument that diversity is important. Which is a good result, because you've made a more convincing study.
But what if the all black team performs the best? It would cause you to look at the study completely different, wouldn't it? Maybe for cultural reasons, black people are better at these kinds of puzzles.
We'll never know because it seems like they have already decided this study in its current form is enough to conclude that "diversity=good".