In short, as a whole women contribute more successful submissions to GitHub than men do
Nope, that is a logical fallacy. Just because the study contends that women are more likely to make more successful contributions does not mean that, in absolute terms, they do. I certainly many, many more PRs from men than women.
Anyway, the logic for identifying the gender of the contributors is flawed. That aside, I can imagine a lot of men perhaps tending to be less harsh on code from women and maybe making changes post-merge or to the PR directly. Call me patronising but I know that I do behave differently towards people who I think may need a bit of help and encouragement than others. At a conference recently I saw a presentation by a woman who emphasised how important it is for women to be encouraged in their work, which would seem to fit this pattern.
Conversely, I can be blunt, terse and even downright rude to people submitting shit.