Anecdotally, I think this underestimates the impact of physical activity. It's much easier to lose weight when I'm physically active. I suspect that this is part of the bodies conditioning response.

Too much fat, particularly fat above the bodies carrying capacity - leads to steep declines in athletic performance. If you suddenly need to shed heat for regular high intensity aerobic work, or pull yourself up a mountain - then its in your bodies best interest to lose the excess weight (up to a point). Our bodies seem to be extremely good at maintaining homeostasis in different conditions, why would body fat percentage be any different?