To clarify, the point is that all else being equal, being fat is not the best mode, rather the fact that one has to consume a large enough amount of nutritionally unsound food on the typical Western diet to become fat does not reflect negatively on being skinny or average or the health and/or longevity benefits thereof, but rather on the poorness in nutrition of said diet types. In other words, if you consume the same amount of nutrients across the whole spectrum of BMI or caloric intake variations, the premise or message of the article would not hold true.

CR has been proven in over 200 species of animals, including monkeys. The final results from human subjects will not be in for decades, but the initial tests (biomarkers) are in line with what would be expected if it were to work with humans.

The bottom line from the CR point of view, is that the American/Western diet is so nutritionally inadequate that it takes a relatively large amount of food/calories to provide the satisfactory level of nutrients that the body requires. That is to say, if you had a number of diets all containing the same level of nutrients, but with just different caloric content, the one who eats a rather low amount of calories, and accordingly had a low BMI, would live the longest. This is the idea behind the CRON diet (Calorie Restriction Optimal Nutrition) -- satisfying your nutritional needs while reducing calories by eliminating non-nutritious calories and consuming only nutrient-rich foods.

I will not make the case for CRON here, the research speaks for itself, and there are many resources for pursuing it online. Just adding the CR view on this type of study, similar versions of which have been reported in other times and places.