It's not a fact, it's a theory. Studies like this one of Greene's may help move it from a theory to something that could be called a fact (i.e., a theory backed with lots of studies). However, if Greene's study didn't control for fluid balance in some way, it's essentially worthless with regards to determining the existence or non-existence of a "metabolic advantage." We might as well just smile and attribute our weight loss success to the Low-Carb Fairy.