At 191 pounds Cahill was still significantly fatter than he should be, and had very minimal muscle mass. We know this is true because it is extremely difficult to not lose muscle on even a small cut, much more so 290lbs over 7 months.

It does not suggest there's something more complicated than calories in < calories burned, it suggests there's something that is causing him to burn fewer calories. My explanation is that Cahill had an inactive lifestyle, poor body composition and was overweight, even after the massive cut. On top of this, no one is claiming he stuck to the diet, so it makes sense he regained the weight.

I'm not disputing that your metabolism can down regulate. I am absolutely disputing that it's a mystery and unfixable. For Cahill he needed to stay on the diet longer, and he needed to build muscle mass.