Isn't that paradoxical? That code is for parsing requirements and those requirements can't be parsed until initial pre-requirements (pip, in this case) are installed.
from my understanding, when you build the package, your computer extracts the relevant required packages, and sends up that data to pypi, which then in turn sends that data back to users when they install
At least thats how I think it works
if you used any other package manager, it would need to resolve dependencies from the additional info in the package index, resulting in pip being downloaded and installed before the desired package is installed