To some degree, yes. Guix is compatible with the low-level Nix package format, and it currently uses the upstream Nix daemon. Everything else has been replaced. Guix uses its own client-side and build-side code. The Nix language is replaced entirely with Guile Scheme, build scripts are Guile programs instead of shell scripts, the UI is different, etc.

On top of the Nix daemon certainly, though not Nix as a whole, which has its own package recipe language etc.