> it does seem possible that caloric excess in the absence of significant dietary fat does not drive obsesity / metabolic disease in the same way.

FWIW this is exactly the opposite hypothesis to that of the Keto diet (whereby consuming fat in absence of carbs does not drive obesity / metabolic disease)

To me seems more likely they were just burning more calories

I think you’re probably right. It is an interesting thing to think about though — since carbohydrate to fat conversion is extremely inefficient, I think you could at least expect a more forgiving change in body composition during a period of over feeding.