I assume (perhaps incorrectly), that if their lowest carb diet tested is still much higher and they're seeing metabolism increases at that carbohydrate intake level, the effect would be more pronounced at even lower (ie keto) carbohydrate intake levels. Speaking only for myself, when I've utilized the ketogenic diet to lose weight, I can lose upwards of 3lbs a week doing nothing (maintaining a sedentary lifestyle) except restricting carbohydrate intake below 20g/day (eating my fill of eggs, bacon, hamburger, and cheese).

This completely ignores any gut microbiome changes (and corresponding nutrient absorption changes that might occur) due to a change in diet.

Yes, you simply consume less calories on a keto diet because you feel less hungry. For whatever reason consuming carbs over a certain threshold is addictive.

Either way, they did not include a keto diet. Instead they used a "relatively low carb" diet of 20% carbs. Since ketosis is an entirely different metabolic state there is no reason to extrapolate from 50%-5% (keto). They have no theory so they need to actually measure this, and every single academic nutrition study I've ever seen on % carbohydrates in the diet fails to do it.