Cosmopolitan is a replacement standard library. It's not so much "pulling-in" a library as replacing e.g. glibc. The point of the blog post is that it's a libc but also a batteries included C development environment. On the one hand, it's nothing magic, on the other hand, it does have some nice concordances which are worth showing off.

This is exactly the sort of drudgery from which computers were supposed to liberate us.