Some things are cheaper off Amazon, yes. But for many people, myself included, it's far easier (and sometimes cheaper in the end) to buy from Amazon anyway, because the stores that sell them may be very far away from where we live. I'd rather pay $5 more and get it in two days than drive 50 to 150 miles each way to a store.
This seems incredibly anecdotal. A box of thirty items from one store in one unknown city picked by an analyst trying to prove something, and all of a sudden Amazon is dead?
On top of that... they didn't factor in time, gas costs, convenience, etc