Just my two cents: Nix doesn’t guarantee “reproducible”, just “repeatable”. (I think it’s helpful to keep these technical concepts distinct, and to introduce the right terminology when teaching, since it makes elaboration easier in the future.)
They should be semantically equivalent, but it's not fully guaranteed.
An upcoming feature of nix is content addressed builds which use the output hashes to determine the store path. This should make it much more transparent which builds are reproducible and which are not.
The amount of time per build can vary. Embedded timestamps will all be in January 1970, but even a single second difference means the binaries are no longer bit for bit identical -- and that assumes the embedded timestamp isn't microseconds.
It's trivial to write non-reproducible Nix derivations by placing timestamps, randomly generated data, or similar into the build output, directly or indirectly. Nix is a powerful tool for working towards reproducibility, since it can specify the software side of a build environment more exhaustively than most other package managers out there, and has a language (derivations) that enables repeating builds easily as well as comparing them, but does not guarantee reproducibility on its own.
A lot of the reproducibility of nixpkgs is attributable to reproducible build efforts elsewhere, including conventions like SOURCE_DATE_EPOCH. Nix itself does not in fact "freeze the clock" for builds, but many tools that use timestamps as part of a build will respect SOURCE_DATE_EPOCH instead of using the system clock. Many of these valuable developments in this direction are coordinated in the cross-distro https://reproducible-builds.org group.
Nix does provide a higher degree of functional reproducibility (builds will have the same behavior, if not identical bits) than other systems do, though, because of the way it locks down exact dependency versions and build flags, though. In that sense, Nix is 'more reproducible' than competing tools even when both are invoking non-deterministic compilers at the last step.
reproducable - get the exact same results every time
not the same thing