He probably already had X11 installed and Nix was downloading duplicate libs.
This is a feature. Nix is completely isolated from the host system. It manages its own dependency tree. Using libs and other things from the host system would be both brittle and nondeterministic, negating the reason why Nix is so novel in the first place.
... except on OSX. Really I just wanted something to take the place of an Arch Linux AUR helper, the way Nix replaces Homebrew.
Ah, I can understand the concern on OSX (or other Linux distros). Earlier in the chain, you wrote "Linux desktop" -- which, to me, implied using NixOS :).
Wayland and Weston work and there's also non-Weston compositors (window managers) like sway, kwin, gnome and more that actually work decently enough with Xwayland integration for running legacy (X11) apps.