Our results show that women’s contributions tend to be accepted more often than men’s [when their gender is hidden]. However, when a woman’s gender is identifiable, they are rejected more often. Our results suggest that although women on GitHub may be more competent overall, bias against them exists nonetheless.
I think the most striking thing is that for outsiders (i.e. non repo members) the acceptance rates for gendered are lower by a large and significant amount compared to non-gendered, regardless of the gender on Google+.
The definition of gendered basically means including the name or photo. In other words, putting your name and/or photo as your GitHub username is significantly correlated with decreased chances of a PR being merged as an outsider.
I suspect this definition of gendered also correlates heavily with other forms of discrimination. For example, name or photo likely also reveals ethnicity or skin colour in many cases. So an alternative hypothesis is that there is racism at play in deciding which PRs people, on average, accept. This would be a significant confounding factor with gender if the gender split of Open Source contributors is different by skin colour or ethnicity (which is plausible if there are different gender roles in different nations, and obviously different percentages of skin colour / ethnicity in different nations).
To really prove this is a gender effect they could do an experiment: assign participants to submit PRs either as a gendered or non-gendered profile, and measure the results. If that is too hard, an alternative for future research might be to at least try harder to compensate for confounding effects.