False advertising, by definition, is the use of false or misleading information, and I think that's misleading to say the least. Although I doubt Nintendo would intentionally mislead in such a petty way, I think it's more than likely a mistake, or perhaps there is a better understanding of this than simply 2 x 0 = 0, because why would Nintendo advertise getting double gold coins when you get zero; that doesn't make sense. I think you came up with this loophole in your mind and now you have yourself convinced, but I think you should rethink your stance on this.
I care so little about this I don't have what would be considered a stance. op asked a question I have my opinion on it and pointed out that the statement in the op is factually correct. Simple fact is they've used the eshop pic for the game which had the x2 points baked into the image. It's a part of the TS&C's of the vouchers that you don't get gold coins with purchases using them because you got gold coins when you bought the vouchers.
I don't mean to sound rude on this post to you or op btw, just on my phone so I am just being direct to the point, there's a Paladins advert jumping in the way of my text and while I love that game I can't stand it's advert jumping in my face ruining the site layout!!!
Fancy hearing me on an amateur podcast with friends gushing over one of my favourite games? https://youtu.be/1I7JfMMxhf8