But you don't see a lot of overweight people there. Even if they don't do anything so don't use a lot of energy I guess calories in are less than calories out so that's why they can't store any fat.

There are other health problems due to this "extreme diet" but being overweight is not one. So if you want to lose weight do like them and eat less than what your body needs.

This is mostly due to malnutrition and not calorie restriction as far as I know. Calorie restriction has many positive health effects. Even then, comparing overweight people to famine struck people is an unfair comparison, it's like comparing a morbidly obese person to someone who is just underweight.