Fat adaptation is not bro science, it is what happens when you do not consume enough carbohydrates to meet your TDEE so your mitochondria “learn” to become really efficient at burning fatty acids. It’s the whole premise behind keto/low carb. When used to our modern high-carb diets, the adaptation takes some time for genes to activate, since we eat a lot and never have long enough fasting periods to be able to quickly switch between glucose and fatty acid metabolism.