This is slightly different thing. Nix and NixOS are trying to solve multiple things, and that's what it might be a bit confusing.

Many people don't realize that, but if you get for example mentioned project from github and I do and we compile it on our machines we get a different file (it'll work the same but it won't be exactly the same).

Say we use the same dependencies, we still will get a different files, because maybe you used slightly different version of the compiler, or maybe those dependencies were compiled with different dependencies or compilers. Maybe the project while building inserts a date, or pulls some file. There are million ways that we would end up with different files.

The goal here is to get bit by bit identical files and it's like a Holy Grail in this area. NixOS just appears to achieved that and all packages that come with the system are now fully reproducible.