Not necessarily: nix the package manager runs fine on other distros, as well as ...

Not necessarily: nix the package manager runs fine on other distros, as well as FreeBSD, OSX, Windows, SmartOS, ...

Nix doesn't need fancy filesystem shenanigans to do its job: build isolated packages with complete runtime dependencies.

NixOS is built on top of it to provide atomic system-wide upgrades and rollbacks.