If it's just a simple additive with a detectable signature, then counterfeiters could obtain the same additive by grinding up real coins. Then they could apply a layer of that material to the surface of their fakes.
So I think, as you suggest, they must (also) be using an anti-counterfeiting method that's more sophisticated than a plain additive. At least an additive that wouldn't return the same signature after it has been reprocessed by counterfeiters.
The problem is, when pressing the coins it might be difficult or impossible to apply such a layer properly.
If the verification is done with a part of the electromagnetic spectrum that somewhat penetrates the coin, then you'll need a very thick, "obvious fake" layer. I am not a physicist so this is just vague speculation.
So I think, as you suggest, they must (also) be using an anti-counterfeiting method that's more sophisticated than a plain additive. At least an additive that wouldn't return the same signature after it has been reprocessed by counterfeiters.