They didn't use very comprehensive research methods. Also they only used Github.

First, from the GHTorrent data set, we extract the email addresses of GitHub users. Second, for each email address, we use the search engine in the Google+ social network to search for users with that email address. Third, we parse the returned users’ ‘About’ page to scrape their gender.

a bias against men exists, that is, a form of reverse discrimination.

How is it reverse discrimination. It's still plain old discrimination. I'm starting to smell a biased research here. Or at least the researchers have a bias.