I think that's also a bit of an unfair comparison given the number of AUR packages you usually use on Arch. With nixpkgs there isn't a distinction between official and community packages.
The nixpkgs are all official packages, it's just really easy to become a maintainer (you make a pull request adding the package you want to maintain)
Reaching for reproducible builds support in Arch is a more attainable goal than for nixpkgs. Properly maintaing 80k packages without regressions is going to be a lot more work in the long term.