That means that you have not disabled cache.nixos.org.
The way the defaults are laid out, is that ["cache.nixos.org"] is not defined as a default value, but as a “flat” value with the same priority as any the values you add yourself. This way one doesn’t have to add it manually when also adding other caches. So “adding” is easier this way, and it is not really expected that poeple want to use nixos without the nixos caches.
You probaly want to use lib.mkForce to give your value a lower priority and therefore precedence over the NixOS provided value.