The study differentiates between male and female only and purely based on physical features such as eye brows, mustache etc.
I agree you can't see one's gender but I would say for the study this can be ignored. If you want to measure a bias ('women code better/worse than men'), it only matters what people believe to see. So if a person looks rather male than female for a majority of GitHub users, it can be counted as male in the statistics. Even if they have the opposite sex, are non-binary or indentify as something else, it shouldn't impact one's bias.