Sounds horrendous and like any of these diet type plans, doesn't teach you anything about real food - nutrition, portion control, calories or exercise - all things you need to have an understanding of for long term lifestyle changes and keeping the weight off.
If you had to lose an amount very quickly say for an operation or other medical reason then I see the appeal of a very low calorie diet. Ultimately though it's just a very extreme version of calories in versus calories out, which works, although a less extreme version will naturally take longer (but in the long term is something that can be maintained in everyday life).
My mum did it about 15 years or so ago. She did the maintence and all and still gained all the weight back and more.
She was adamant at the time it was going to be the last diet she would ever do blah blah. She went weekly to the ‘food therapy’ or whatever they called it to help you address your food issues, and nothing worked tbh.