[nl]

I think this is the most important part, and provides the best explanation of the results:

part of that finding can be explained by differences in social sensitivity, which we found is also important to group performance. Many studies have shown that women tend to score higher on tests of social sensitivity than men do. So what is really important is to have people who are high in social sensitivity, whether they are men or women.

So it isn't men being more competitive - it's that teams with people who work well with others generally perform better. On average women are more "socially sensitivity" so more women will, on average, imply a better team.