There's a large drop for both men and women between project outsiders who do not identify a gender on GitHub and those that do. So that's one very clear effect. If you want to have your contributions accepted on GitHub, male or female, and you are an outsider, do not identify yourself.

However, it was only possible to identify the gender of about a third of people overall from publicly available information.

So for the conclusions of the study to be valid, one must eliminate the possibility that there is a gender imbalance in those whose gender is able to be identified by publicly available information (which would not represent discrimination, since that information is self-reported). Until one can eliminate this, one may not be measuring discrimination, but effectively just a correlation between availability of public information on programmers and their skill level.

Moreover, it looks like the gender information was entirely constructed from Google+. Yet there is no information on what percentage of users' of each gender actually had Google+ pages. This is extremely relevant to the conclusions as there could also be correlations with the usage of Google+ by programmers of a certain skill and gender.