They probably don't have RFID sensors on the shelving, and if they did they would only be able to detect when you walk several feet away from the shelf, and would have huge problems if two people were standing right next to each other at the shelf.

It looks like they have a row of cameras along the top of each shelf that will be used to detect when you pick up and place items back down.

The RFID tags are mostly useful at the gates to confirm the visual data and feed back into the machine learning algorithm.