In fact, by every metabolic measurement, they seemed like people who were starving.
ADBOC. I don't know that seeming like someone who is starving, so long as you aren't actually risking to die from starvation and your micronutrient intake stays adequate, is a bad thing, and indeed the evidence seems to suggest that it isn't.
And yeah, slowed metabolism means that if you go straight back to eating as much as you did before starting the diet, you'll gain back the weight. Which is why people are usually advised not to do that.
The researchers concluded that 70 percent of the variation in peoples’ weights may be accounted for by inheritance,
Controlling for height and sex?
Link to twin study. A quick scan (I don't have time to read it in full right now, but I will later) suggests they used twins of the same sex, and they also compared BMI not weight, which controls for height.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.