Ok, first off, that is NOT an AT&T advertisement. What you directed us to is a 3rd party blog in which someone gives his interpretation of the deal. If you'll link us to the actual AT&T ad, I'd like to read the fine print (there is ALWAYS fine print).
Second, if it is to be believed, it clearly states data plans between 15 gb and 50 gb are eligible for double data. I believe you've said time and time again, you have (and are paying for) a 10 gb plan, for which you receive an additional free 5 gb.
{keep it courteous}