“Nix” is a very overloaded term. It can refer to Nix-the-language, Nix-the-interpreter, Nix-the-command-line-tool etc. CppNix is specifically the code body developed in https://github.com/nixos/nix. It (among other things) implements Nix-the-language. E.g. https://tvix.dev/ also implements Nix-the-language, despite sharing no code with CppNix.