Yes, I agree that reproducible builds is something that can be solved fully independently of Nix, and that Nix alone does not solve it, and that as such it is independent of Nix.
However, as the point of Nix is to achieve reproducible software deployments through a pure and functional description of the deployment, where it also provides mechanisms that systematically improves build reproducibility, I feel that orthogonal is misleading (as was my original disagreement) because reproducible builds correlate with Nix, and because achieving reproducible software deployment is a clear original goal of Nix, e.g. by seeking to remove implicit dependencies through mechanisms such as sandboxing of builds.
However, as the point of Nix is to achieve reproducible software deployments through a pure and functional description of the deployment, where it also provides mechanisms that systematically improves build reproducibility, I feel that orthogonal is misleading (as was my original disagreement) because reproducible builds correlate with Nix, and because achieving reproducible software deployment is a clear original goal of Nix, e.g. by seeking to remove implicit dependencies through mechanisms such as sandboxing of builds.