Earlier quoted context omitted.
Roughly, the method used is the ketogenic diet. Very low carb, very high fat.
It might not necessarily be high in fat - it's probably the calorie restriction that works, as in this trial: https://www.nhs.uk/news/diabetes/radical-low-calorie-diet-ma...
Personally, and anecdotally, I have tried calorie restriction dozens of times since childhood and it has never brought down my A1C.