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.
I'm guessing the Amazon Go app generates a one-time-use QR code that gets scanned on entry and then security/tracking cameras follow you all over the store. If the cameras see you in front of an item when its RFID sensor detects it was picked up, they make an educated guess that you picked it up. Then they can re-scan all the RFIDs as you exit for extra confirmation.