Nope, they were using camera technology and QR codes
Even if you couldn't do this at the exit, seems like it would have been a far easier lift to incorporate the same idea into shopping carts or baskets all along.
You'll note that WalMart doesn't even use the EPCs at POS, which is telling: for most retailers, the main advantage of EPCs is far more actionable alarms at the exit. So they're limited to items where loss rates make the added cost worth it.
The problem is that the grocery industry has notoriously low margins, and the unit price of EPC tags can be the entire margin on a lot of products. On the one hand, Amazon may have been trying to work around the need for higher-cost tags to roll out this kind of automation. On the other hand, I have heard anecdotally that Amazon Fresh pricing was relatively high, so maybe EPCs would have been a wiser use of their extra revenue.
But that said, interesting point. Didn't know the cost of an RFID tag was that high -- after all, Uniqlo is putting them on some pretty cheap clothing items!
For apparel, on the other hand, source tagging is common - even before EPC on higher end goods, Calvin Klein used to sew magnetostriction tags into clothing. Apparel just has so much more price elasticity and loss prevention is a huge part of that industry. Tools are another industry where EPC and source tagging are common, once again, high dollar items with a lot of theft.