Technically, it's less than 1 calorie for every 200ml ng diet soda. Of course, if you're drinking a lot, the calories will eventually add up, but it's still very low.
Yes, it's probably a marketing strategy, but it doesn't mean it's all bull. It all depends on your diet before you switch to diet cola.
Kung masaya ka nang umiinom ng tubig lang before, then switching to diet cola will maybe give you another 1-2 calories each day. But for most ordinary people, malaking kabawasan sa calorie intake yung pag-switch to diet drinks.