I mean the method used by Virta in this particular link. I am not suggesting that there are not other methods that work.

Personally, and anecdotally, I have tried calorie restriction dozens of times since childhood and it has never brought down my A1C.

It must be high fat. From the linked essay [1], the diet is 30g daily carbohydrates, 1.5g/Kg(body mass) protein. For an 80Kg individual, that results in 150*4=600Kcal. That means almost 2000Kcal in fat to reach the daily caloric intake.

[1] https://asset.jmir.pub/assets/a2c0047f60bd77156d22029b8bdd5c...