Whenever you see a "high-fat diet" study, you can usually translate that to "hypercaloric diet" study, and this one is no exception.

One fly food was 8% glucose (.308 kcal/g) and the other was 8% glucose + 20% coconut oil (.308 kcal/g + .7136 kcal/g = 1.0216 kcal/g) leaving the high-fat arm with 332% of the caloric density of the standard fly food diet. The discrepancy clouds how much of the observed effect is due to the coconut oil, and how much is due to the difference in caloric density.

I looked into how well fruit flies self-regulate caloric consumption, but all that I could find was related to carbohydrates and protein. It would seem that fruit flies don't typically eat much in the way of fats, presenting another confounder in the results.