The arxiv paper [0] shows some further examples, which include some non-white faces. Your point is a fair one in the case of this application, since the goal is to do 3D reconstruction of the face. The project's YouTube video [1] shows one example (around 0:44) of the reconstruction run on test input of a black woman, and the reconstruction has some troubles modeling the wider nose common in black populations. More varied data (or an ensemble method that attempts coarse racial categorization and then further refining ethnicity-specific face models) might get better results.

[0] https://arxiv.org/pdf/1803.07835.pdf

[1] https://www.youtube.com/watch?v=tXTgLSyIha8&feature=youtu.be