That’s false. Carbs are not a requirement for human body. Proof of that are native populations living far north, exclusively off of animal food sources.

Human body prefers ketones as a source of energy. But glycogen is faster.

My simplified mental model is like this. Ketones are normal gasoline. And glycogen is racing fuel (ethanol). If you run a stock car on racing fuel it may start and drive for a while but it’s damaging the engine and creates unnecessary wear.