Lack of sunlight is usually also correlated with cold - and being exposed to cold makes you burn fat fast, to maintain your body temperature. Which might be the reason for the finding in the first place.
But the article is proposing the opposite. "Lack of sunlight makes our cells store more fat."
I would say that being in a cold place makes you eat more. And being only sporadically exposed to the cold could somewhat trigger this evolutionary instinct of storing energy without the expected proportional increase on energy expenditure. Long term, fat accumulates.
