They studied a number of things and "... when they controlled for the number of women in a group, it was shown that it was the emotional sensitivity scores which won out."
Whatever its merits or flaws, the study was not simply about adding women to groups of men.
Initially, they discovered that the most significant factors were "the high average social sensitivity of group members, a high rate of sharing who gets to communicate, and more females". Then they controlled for the number of females in each group, and discovered that the number of females actually didn't matter so much - the women in the study were just more likely (than the men) to have high social sensitivity, so a team with many women would be more likely to do well. But a team consisting of all men would also do very well, as long as you made sure to pick men with high social sensitivity.