    [user@nixos:~]$ which grep
    /run/current-system/sw/bin/grep

    [user@nixos:~]$ ls -l /run/current-system/sw/bin/grep
    lrwxrwxrwx 1 root root 65 Jan  1  1970 /run/current-system/sw/bin/grep -> /nix/store/737jwbhw8ji13x9s88z3wpp8pxaqla92-gnugrep-3.12/bin/grep

Basically, it is still in your environment, so I don't see how he can claim that this problem doesn't exist in Nix, unless you use flakes like a proper Nix afficionado.