As a layperson? Magic, because that’s how these things are marketed by startups and journalists.
It produces no black faces. Even as a guess.
Though, things like black people being more poorly recognized by facial recognition can be a blessing in disguise depending on the circumstances
That, by itself would be biased for the ML, since there are fewer images. But it's not an indication of bias by the people programming it.
But this would imply you need huge data-sets for every minority, not matter how small a proportion of the population.
It might instead be necessary to teach the model about race as a concept, so it can categorize images, and then process them correctly. But of course that leads to a different can of worms since you are explicitly making a "race aware" ML.
If I want to work on a project like this, and the only appropriate training dataset I have is photographs white males, am I not allowed to work on generating faces until I've fleshed out the data set?
It's not like they are offering some service to the general public where they can de-pixelate faces. It's a research paper.
A final note: The only professor with her name on the paper(Cynthia Rudin) is very active in researching the intersection of machine learning and social justice. I'm not so sure she would put her name on a paper that can be flippantly described in 3 sentence comments on internet forums as having "some issues" wrt race.