Those two files do have a low ratio of comments. However a large number of files in those code bases have quite a lot more comments... I personally don't think these two code bases have sparse comments, overall, although perhaps it's subjective or my bias is affecting my judgement :)

Eg, to me this is well-commented and I wouldn't call it sparse.

https://github.com/antirez/redis/blob/unstable/src/latency.c