The 'study" was based on BMI.
If you ask ANYONE in the fitness industry, they will tell you BMI is the biggest load of horseshit in the world, and might be the worst indicator of health ever invented.
Thus, the study and article is complete bullshit and worthless.
There is not one reputable study that says being fat will help you live longer, because it doesn't for so many reasons I wouldn't have time to list them all. Just the opposite actually.