It needs to be explained how he determined that this isn't related to _why_ they are obese in the first place
I agree that more explanation is needed...the thinner person may be thin because she is more active, and the obese person may be obese because she is more sedentary. An extra 10 calories would seem more likely to put more weight on a sedentary person than an active one.
The reverse claim has just as much explanatory power. That is to say: The active person may be active because she is more thin, and the sedentary person may be sedentary because she is more obese.
I agree that more explanation is needed...the thinner person may be thin because she is more active, and the obese person may be obese because she is more sedentary. An extra 10 calories would seem more likely to put more weight on a sedentary person than an active one.