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)