[iplaw]

I think that he is referring to the immediate weight loss effects associated with depleting the glycogen stores in your liver (400 grams) and muscles (100 grams). Each gram of glycogen is bound to 3 - 4 grams of water.

Thus, depletion of your glycogen stores results in the loss of 2.0 - 2.5 kilograms (4.4 - 5.5 pounds) of "water (and glycogen) weight" within the first week of a low-carb diet. Larger individuals often retain even more glycogen and water, which can result in immediate losses of 6 kilograms (13.2 pounds) or more.

His point is that some studies, particularly short term studies, do not account for this non-fat weight loss.