In the early weeks, I even tweeted: “Dieting is not so much about losing weight as it is about gaining an eating disorder.”

This is what they actually think. Eating too much carbs too often and making yourself insulin resistant "not an eating disorder." Eating a proper human diet like a proper human "an eating disorder."

because being fat is the worst kind of failure.

It literally is. You're literally failing to maintain your actual SELF. What could be more of a failure?