>> "Although in Mongolia the diet has traditionally been extremely fatty, the extreme cold climates and the hard physical activity of the Mongolian people has meant that obesity has not been a major problem in the country, although it has had a high rate of prevalence among foreign workers in the country, such as European Russians, many of whome were involved in a more sedentary lifestyle."
I think they're probably referring to the fact there is some evidence that exposure to cold promotes "brown adipose tissue", a metabolically-active type of fat. Mark actually has a blog post on this: Is Brown Fat Good For You? | Mark's Daily Apple
But of course, as you point out the problem is probably not their traditional fatty diet but rather the large amount of processed junk that's coming from abroad.