The demo in the repo only shows inpainting of anime-style geometry. In the 4chan thread linked by nathansmith, someone tried to apply it to a mosaicked penis, but it disappeared (NSFW, probably: https://i.4cdn.org/g/1540580868203.png). When they used the mosaic decensoring mode instead, it worked slightly better (NSFW, definitely: https://imgur.com/a/PwAunbc/). Other people complained about missing genitals as well.

So I'm guessing that this model doesn't have any specific training for filling in only genitals. I think there are category-aware inpainting algorithms that could be used, but they'd need tagged data, as you say.