I think the poster was implying that you trick the drone into delivering your neighbor's package into your backyard by putting the card out even when you're not expecting a delivery.
GPS is way more than accurate enough to discriminate between your backyard and you neighbours. Even with poor satellite geometry I rarely see more than 1 or 2 meter CEP when I'm up above the tree/building line.
I'm betting that by the time this is real (and assuming the shown implementation is what actually goes live), everyone will have their own personal QR code so there will be no mistakes.
Did I say anything about keeping or even opening the packages? If the drone is using some sort of convnet or HOG to detect the landing pad, I can only imagine hilarity would ensue if its visual field were saturated with multiple copies of the target image, a sort of visual DDoS if you will...