I think it is the stupidest thing I have read today (and I read a lot today).
But, let me elaborate (the fun part).
Picking a number is the problem - 800 - where did that come from???
For a small (130 trying to get to 120 pound sedentary) woman - this definitely could be viewed by the body as a new norm that it should try to adapt to - defend fat stores, slow the metabolism, spike insulin etc. All the bad things happen - horrible diet.
For a large (200+ pound - only 10 pounds to lose - 6 foot 4 inch muscular, athletic and regularly trains) man - 800 calories be so little that the good fasting adaptations of increased HGH and metabolism will kick in. Making this a great diet for him.
The calorie amount (and macro breakdown) of a daily food intake that is at the limit of what causes good fasting adaptations vs. what causes metabolic problems is (as far as I know) unknown. But, I am sure it would be related to body weight, lean body mass and level of exercise.
As far as regulating blood sugar - probably the same - good for the man, bad for the woman (in the above examples).