My guess is that potato diet guy is under large caloric deficit, the body needs minimum calories to run basic functions, once those are depleted it starts using

My guess is that potato diet guy is under large caloric deficit, the body needs minimum calories to run basic functions, once those are depleted it starts using the stored fat. But my understanding of ketosis is that it is a metabolic state during which the body uses the stored fat as a fuel and ultimately all roads must end up at that state for the stored fat to be depleted.