Nix (the package manager) does not depend on systemd in any capacity; just like apt, apk or Pacman do not depend on systemd.

NixOS currently uses systemd and cannot be ported without major restructuring. So far the enthusiasm to do that has been very limited. InitWare may be the path forward but that project seems to have stalled.
Note that NixOS also does not support any kernel other than Linux either.