This is misleading. The fact is that Diet Coke has run afoul of the FDA's Label Mafia. DC isn't lying when it says it has added vitamins and minerals to its "Plus" product.
To the average consumer, "plus" is a reasonable description for a product that is diet soda "plus" vitamins and minerals. To the FDA, which has labyrinthine regulations for just about every word that appears on a product label, it's a mortal sin.
I think it's also interesting that (according to SciAm) the FDA "does not consider it appropriate to fortify snack foods such as carbonated beverages." It's just a matter of time before "does not consider it appropriate to x" will become "forbids x". Just you wait.
The FDA should be abolished.