I don't think that is quite right. The body doesn't discretely switch from one fuel source to another. It may favor fat vs. BHOB vs. Ethanol vs. protein vs. carbs vs. Acetone but it's not like it "decides" not to burn fat or something. A good example is the Phinney an Volek study which showed carb infused bicyclists raised their ketones from <0.05 to 0.46 through exercise. The only way to increase ketones is to burn fat so these carbed up athletes were burning fat.