That is correct. It is more dangerous to be underweight than overweight, especially in the short-term (although we should probably draw a distinction between "slightly underweight" and "very underweight" -- but still, 30% overweight is not a crisis, 30% underweight definitely is).
If someone is overweight and all their numbers check out well (blood pressure, cholesterol, insulin sensitivity), then they ARE healthy.
That said, I think Kate Harding is missing one thing... the fact that obesity has actually increased in the last several years. She's probably right that it's genetically determined, i.e., I ate a SAD for my whole life until the last couple years but I'm obviously genetically thin. But for a large number of people, a "normal healthy diet" by modern standards causes weight gain. (Personally, I think that doctors, nutritionists, etc. recommend WAY TOO MANY grains, and that's the reason... I don't think it has much to do with junk food, because although yes there are junk food addicts, there are also plenty of people who eat "healthfully" and still can't lose weight.) But in some people that coexists with disease and in others, it doesn't.