1) Exogenous Ketones are legitimate, but in an endurance sport - your body will burn the ketones as energy before whatever carbs your body has stored - so you don't "hit the wall" as early.
2) Ketosis and a Ketogenic Diet is also legit, making your body burn fat instead of carbs, when combined with a calorie deficit, is a tried and true method for weightloss and managing hunger.
Taking Exogenous Ketones when not in the Tour de France (or actually exercising at all) doesn't exactly make sense. They are used to increase your athletic performance! It makes as much sense as sitting on your arse drinking protein shakes and expecting to look like Arnie.....