I think we should distinguish between the graphics that are generated by “pygame.draw…” instructions, and the possible use of picture files.
The generated graphics are free for sure.
But the picture files always can be covered by various rights (the copyright is not the only right that might affect us).
Critter&Guitari do not deliver picture files, so they can say, that the results are free.
But as soon as you will use image files in self written modes or downloaded modes, you are responsible to assure, that these images are free (at least as soon as you get into the public with the ETC-projections)