This is a fork of the evaluator/language implementation/daemon/builder/whatever you want to call it. The other one (Auxolotl) is a fork of Nixpkgs, the repository of build scripts and all the NixOS misc pieces.
Or put into other terms, this is a fork of APT/RPM as well as their associated builder tools, while Aux is a fork of Debian/Fedora/whatever. The Nix evaluator is a much more complex piece of software than most other package managers so it does benefit from having a dedicated team working on it.