No, it wasn't actually...
"Cooperation favors same-handedness—for sharing the same tools, for example. Physical competition, on the other hand, favors the unusual. In a fight, a left-hander would have the advantage in a right-handed world."
This is simply taken axiomatically as a starting point for the study. I see no indication that it was determined by any sort of analysis. I'm not even sure such an analysis is feasible. You'd have to know what activities people carried out in prehistoric times, how, and what the value of cooperation was for each one. There could be various advantages and disadvantages of same or opposite handedness depending on the activity, etc. The entire concept of their study rests ENTIRELY on the validity of this same-handedness is better for cooperation proposition. I'm not saying it is untrue, but without demonstrating it to be true and to what degree I cannot see how any meaningful conclusions can be drawn.