That's how Git submodules [1] show up on GitHub. I didn't actually know the project was using them. Interesting. As far as I know, this is an ideal use case for submodules, yet at least I've rarely encountered them in the web dev scene for some reason (well, IIRC there are some gotchas).
[1] https://git-scm.com/book/en/v2/Git-Tools-Submodules