"The investigators discovered that given a chance, and in the presence of a low-fat diet, bacteria from a lean twin will take over the gut of a mouse that already had bacteria from a fat twin. The fat mouse then loses weight. But the opposite does not happen. No matter what the diet, bacteria from a fat mouse do not take over in a mouse that is thin."
Second, the context (described later in the article) is among co-habitating mice that eat each other's droppings, allowing their gut flora to commingle. When the mice were kept separate, the mice that had received gut flora from a fat human twin ended up 10-15% heavier even on the identical low-fat diet as the mice that received flora from a thin human.
(Aside, because it's personal: I have no trouble controlling my weight through diet and exercise. When I ate poorly and exercised little I was as heavy as 190 lbs. With exercise and being mindful about my diet I have no trouble maintaining 150 lbs. My wife, otoh, has struggled with her weight almost her entire life. To lose weight she has to weigh and track every ounce of food she consumes. It is a constant battle for her. Sadly, I see this dichotomy playing out in our son and daughter. My son will never struggle with his weight; I fear my daughter always may. And we eat well in this house. Real food, mostly plants, not too much.
So yeah, I think some people are thin almost no matter what they eat, some struggle with weight almost no matter what they do, and some are more in between. But a holier than thou attitude about being thin helps no one.)