Maybe I'm just a cynic, but I'm thinking...if a bra can handle "back-fat" issues...then you probably don't actually have back-fat issues. Your current bra is just potentially a bit too constricting. In any case, I don't believe that by purchasing this bra, anyone will actually find themselves saying, "Goodbye!" to their extra back-ness.
So I'm thinking "Bye-Bye Back Fat" would probably fall into the "empty promises" category. Or, if you're one of those people who files lawsuits over ridiculous things, perhaps "false advertising".