He does also say that the 800 calorie diet can be done in the same way the original 5:2 was/is. Namely, 800 calories two days a week. The 16/8, low carb 12 week version is only for the masochists or diabetics.
Although low carb is very popular, there has been a research published recently suggesting that in the long term this eating pattern can have serious health risks. www.sciencedaily.com/releases/2018/08/180828085922.htm
I know a number of GN ers do the 16/8 daily eating pattern and have found it very successful, without any of the add-ons. The 800 a day for 12 weeks diet is specifically aimed at those with diabetes as this diet is claimed to 'cure' diabetes. But less extreme versions of this diet have also been effective against diabetes. DH, who has/had diabetes, did the original 5:2 with me for about 4 months about 4 years ago . At his next diabetes check he was told that he no longer needed medication and he has stayed at this level ever since, even though subsequent health problems meant he regained all the weight he had lost (and more).
At the end of the day, it is a question, if you need to lose weight, of finding a diet pattern that suits you, and your life style.