I disagree. I was recently in the transmitter room of a radio station and saw an interesting device. I asked the engineer what it was, he explained it was for ratings and worked exactly in the way the article describes. They transmit inaudible tones on the radio, and they are picked up by certain people with a device or the app installed. I don't recall all of the details but some googling will probably get you going.

They use this to determine how many people are listening to their station.