The article has some answers to how they can do it (advertising data, for instance). As for what happens if you deny it, well, you're not in a court. They don

The article has some answers to how they can do it (advertising data, for instance). As for what happens if you deny it, well, you're not in a court. They don't have to prove it to any evidentiary standard and you may not even know why they decide to call you to the separate room for additional screening.