The polybar package in Nixpkgs looks correct to me, and so does your usage of it.
Perhaps the binary that you’re executing is not the one you think it is. Check /nix/store/ for other derivations of polybar and see if any of them are correct; ls /nix/store/*polybar*
So it seems to work as it should. Are you sure you’re not installing another version of polybar in some way? Via the home-manager module, users.users.<username>.packages, nix-env --install, or nix profile install?
I woke up and received problem-solving skills previously unavailable to me. @olmokramer was also right about that. Except I didn’t really specify any package, but home-manager still used the wrong package. What I had in a home-manager module is services.polybar.enable = true, which seems to ignore what I had in configuration.nix. What I replaced it with is: