Wait it's integrated with SystemD? What is a package management system doing with the init process? I now remember that Guix is a daemon, something that made me really wary at the time; but a SystemdD dependency is something else entirely.
NixOS can be validly viewed as a very fancy Systemd configuration toolkit.
If you think about it, ultimately an instance of a modern general-purpose OS is structured as a set of services which are described for and orchestrated by, well, something. In the case of NixOS (not Nix in general!) that something is Systemd (which also happens to be used by all other major Linux distros); in the case of GuixSD that something is GNU Shepherd (which also happens to be used by GNU Hurd)
Nix is also generally run as a daemon. It doesn't have a dependency on systemd and it does run on MacOS. I think also it runs on FreeBSD in principle.
The parent poster was probably conflating NixOS and Nix. NixOS is a way of using nixpkgs to build/configure and install/upgrade an operating system based on Linux and systemd. But you are not obligated to use systemd just because you're using nixpkgs.
NixOS uses sytemd for essentially the same reason other Linux distributions do: most user environments are now built with the assumption that systemd underlies them if Linux does as well. In principle you could fork nixpkgs and make all the necessary changes to remove the systemd assumption - managing a NixOS system and hacking on nixpkgs are basically the same skillset so it's not like managing an Ubuntu system where you would have to become familiar with how Ubuntu gets built. With nixos, the command is the same whether you're building an entirely new distribution based on a custom fork or whether you're just downloading builds of the latest security upgrades. Aside from the actual energy it takes to maintain a second system manager, you would only have to wait for the builds to complete rather than learning how to build everything. But so far no-one who has tried that has got enough traction - the cost-benefit just isn't there. Honestly, I think this is part of the reason people are offended by NixOS in particular using systemd: it feels like it should be possible for you to remove the systemd dependency, but you haven't done it.
