If you make to the end of the article, they do account for exactly this issue by comparing cases where they could find an out-of-band gender indication (google+) but where the name was not identifiable via github name/profile.

From the article:

> For gender-neutral profiles, we included GitHub users that used an identicon, that Michael’s tool could not infer a gender for, and that a mixed-culture panel of judges could not guess the gender for.