PIN on the back
This makes no mention of the security feature that is on every gift card I have ever seen, the PIN on the back which you have to scratch to reveal.
You shouldn't be able to check the balance or purchase anything online without that PIN. Which means their attack would only work in physical stores, which with the amount of CCTV and loss prevention teams would be a bad idea, especially if you have to guess the last 3-4 digits(1 check digit).
You should never accept a card with that PIN already scratched off as it means someone could go online and use the credit. Someone could grab a load of blank cards from the counter, take them home, read the cards and scratch the PIN off, then go back and put them in the store and just wait for them to be loaded up.
Staff are supposed to be trained to check the cards haven't had the PIN scratched off before loading them up.