I wonder if their data will scale? Is it effected by temperature changes? Humidity changes (especially Gen2 tags)? It's one thing to notice the uniqueness of a few hundred chips, but it a passport database could have billions of entries, or say a database of tagged cash with trillions of entries, would entries still be unique under varying temperature and humidty? Or just mostly unique, like social security numbers?
Another way of reducing counterfeiting is to track where the item is supposed to be in a secure database (or secure databases linked by secure communications) and if the tag shows up in an unexpected place, investigate further. In the passport example from the article, if passport X is known to be in the US and its counterfeit tries to be used in France, that should trigger further examination. Of course, this requires all the passport computers to communicate world wide which could be administratively difficult, but probably not a lot more difficult than figuring out which database of response curves to query.