When I said no legitimate company gives a 15% discount on Disney gift cards, I was talking about a straight discount - you buy a $100 Disney gift card and nothing else and they only charge you $85.
This is different than getting money off your groceries or gas points where the value might be 30% to one person but different to someone else. For example, Shoprite a couple of times a year offers $10 off your next shopping order when buying a $50 Disney gift card, worth 20%. But that is not a straightforward 20% discount, as the value of the $10 off might be worth 20% to one person but less to someone else who would be able to buy those same groceries at another store like Aldi's or
Walmart for cheaper.