I just watched the video. They mention that they use equipment from http://www.aerohive.com/, which just seem to be normal wifi infrastructure.

It seems like only people who actually connect to the wifi can be tracked. There's no way to harvest the MAC address of a wifi device that hasn't connected to your network, is there?

As a store, this seems to be not that valuable to me. I can't imagine that very many shoppers actually take the time from shopping to decide to connect to your wifi. I get that they only need to do this once, and after that, it will auto-reconnect. I'm still not buying it as an effective tracking method.