Unfortunately, nix does not produce fully reproducible builds. The build environment is portable and produced in a way that it can be repeated, but due to the limitations of the software that is being built, the builds are not binary reproducible. You can see some commentary on the nix team hoping to adopt some of the work being done by debian et al here: https://github.com/NixOS/nixpkgs/issues/9731

NixOS currently isn't there. A lot of this work is done, but much still remains. We have benefited greatly from Debian's work, though (Debian maintainers frequently come across as happy upstream participants to fix issues like this in the ecosystem, which really helps everyone!)

https://r13y.com/ tracks the progress of NixOS reproducibility; currently we're at 98.23% bit-for-bit identical for our minimal installer ISO. After that, we'll need the graphical installer, and then more of the base package set. So we've still got a ways to go.