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.