TFA lists C at number 3 per the Tiobe ranking and the linked Tiobe report has Ruby at #24. I tried to find a different list for comparison as I dont think that is true for projects at large. This project [1] for instance tracks github repositories with Ruby at #8 and C at #9 which is closer to what I expected (both in the top-fifteen, neither in the top-three). At least it confirms my bias, and maybe we learn more about who is being surveyed.
My understanding is that TIOBE is primarily based on number of search engine results. Other results like number of active (a commit within the last 6-12 months) open source repos or number of new repos in a given language are other metrics worth considering. The project you linked seems to be one like this.
A high search rate doesn't necessarily mean high rate of real world usage. Correlating across multiple metrics would be a better way to measure popularity.
Although popularity itself may still be a weak signal depending on your purposes.
[1] https://madnight.github.io/githut/