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.