You haven't read r13y properly.
>1733 out of 1737 (99.77%) paths in the nixos.iso_minimal.x86_64-linux installation image are reproducible!
600 packages becomes 1737 paths which are the inputs used to generate the image. Several of the compared paths are just static files which doesn't change, so the "path to package" comparison doesn't work either.
EDIT: An example set of stuff it attempts to compare can be found here:
https://paste.xinu.at/TmVM0y5FBDWX/
Taken from the build here: https://buildkite.com/grahamc/r13y-dot-com/builds/1149#01813... They have not solved the problem.