Apparently, he did a mono diet to break what he called an addiction to higher calorie junk foods. Literature suggests that diets consisting of only a single food source actually retrain the brain to interrupt cravings for sugar, salt, and fats found in foods people over-indulge in.
I suspect that a ketogenic diet wouldn’t have preserved too much more muscle. Keto is high fat, low protein. When you start eating too much protein on it, the liver converts the aminos to glucose via gluconeogenesis, so it wouldn’t be used for tissue repair anyway.
Keto is muscle sparing. When you are keto-adapted, the brain's demand for glucose decreases greatly. The majority of those demands are met by ketones in the blood. Also, the body will more readily oxidize fat for energy, in addition to ketones, which lowers overall glucose demands of the body greatly.
With lower demand, there is much less need for catabolic breakdown of muscle protein to fuel anything, because most functions are fueled from fat or fat-derived ketones. You do however have to be keto-adapted. When keto-adapted, glucose is no longer the primary fuel, therefore demand decreases, as does catabolism of muscle tissue.
That is not to say that you won't lose any lean mass. You will lose muscle glycogen and the attached water, so your lean body mass will indeed go down compared to other types of diets. However, glycogen and water in you muscle is not actual muscle, but it does make your muscles appear larger. Most of that mass can be replenished via strategic carb loading.
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