Comment by lloeki

In a sense, nixpkgs is that.

And AIUI that's exactly that the author refers to by "95% is nixpkgs".

The NixOS part (the module system and modules) is in no small part integration between the init-agnostic nixpkgs and the init system (systemd).

That's what enables this project, as well as nix-darwin (integrates with launchd) and NixBSD.

The traditional way (pacman, apk, apt...) is for packages to carry the init files for the service they package.