That's a good question. I don't know why you are being downvote. How does self control correlates to obesity? That can only be the case if we are talking about a narrow way of self control.
Wasn't the metric for calculating self control was how long you were willing to wait for an additional marshmallow? Seems like a proxy for measuring self control that would be biased towards people with a propensity for obesity.
Anyway, the article is doubtful at best.