That is exactly what their study concludes: "One explanation is survivorship bias: [...] Another explanation is self-selection bias: [...] Yet another explanat

That is exactly what their study concludes: "One explanation is survivorship bias: [...] Another explanation is self-selection bias: [...] Yet another explanation is that women are held to higher performance standards than men [...]" Or in other words: Women who stay long enough to end up submitting PRs submit higher quality code, while the much bigger population of men, under much less social pressure, has a more well-distributed quality curve; and even women who get this far are biased against if their gender is known.