Earlier quoted context omitted.
Guix, and I think Nix also, package the things from these other package managers so you can manage them all with the same package manager. This also means features like rollbacks can apply to your emacs packages. You can run pip on Guix System, but I don't think you'd have to, ideally. Same for rust's cargo and so on.
Ok, makes sense. Does that mean that you don't get the latest updates that are available in pip? Also, what happens if you incidentally run pip on a Nix system? Will it mess up your installation?
[0] guix install mpv --with-commit=mpv=cc4ada655aae06218b900bb434e3521566394cde