It's a shame there isn't a distribution (at least that I'm aware of) which is dynamic and modular enough to allow choosing your own "process manager" at install or even on boot.
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.
Portage is the best package manager out of any distribution.
There are no buts. Gentoo is awesome.
...granted, my potato only has an i7-6700 Skylake with 4 cores+hyperthreads for 8 threads at 3.40GHz, it's not one of those AMD Ryzen things, but still: I'm not running GNU/Linux because I want to buy a new PC every year.
Antix is essentially Debian for Dummies on crappy hardware running live in RAM, or otherwise unorthodox ways. It may seem ghettoish, but the thing about it is you can mod/remaster it most easily and make it YOURS without much fuss. And because of running from RAM it really flies.