I think they're referring to this implementation: https://github.com/nix-community/nixos-anywhere

but cleverca22's work is solid as well

there's also the earlier https://github.com/elitak/nixos-infect

and the built-in NIXOS_LUSTRATE: https://nixos.org/manual/nixos/stable/#sec-installing-from-o...

I'd go with nixos-anywhere, personally