We don't eat granola to clean our teeth but we are capable of thoroughly brushing our teeth 2-3 times a day every day. We can even use mouthwash and floss. SOme cats don't even allow brushing once a day.I will never understand this. My response when I hear that idiotic nonsense is, "Do you eat granola to clean your teeth?"
The research shows chewing hard food helps cleaning the tartar. And in the case of eating wet food the food sticks to their gum (especially pate food) and a hard food removes them off the gums. It's not the ideal method but the ideal methods don't work well and/or are not enough with every cat.