Nothing in nix says you have to compile something from source. Just that the resulting artifact needs to be reproducible hermetically. You can download any random blob from the internet as a nix derivation, as long as you tell nix what the resulting hash should be after you download it. Sure, it might have unmet runtime dependencies, but thats orthogonal.

What you are really butting up against is that the nix store is a bit of a split-brained runtime environment. Its not easy to e.g. `gem install` to your system while running a nix-managed ruby. This has nothing to do with the binaries (well... sometimes it does because nix will patch paths to point to the readonly store, but again thats orthogonal).