What most of these people have is indeed an eating disorder, but it's food addiction. And like any addiction, it's hard to break, but the first step is to admit you have a problem, which is the step a lot of these folks are missing.
It's possible that what they're referring to is one of two things; first of all, it's quite easy for morbidly obese people to actually be malnourished; they eat so many fatty, starchy, oily foods that they don't eat many fruits and vegetables, resulting in a massive body that's massively deficient in basic vitamins.
The other is that many do get hugely obsessive about their eating habits and their attempts at dieting, but they go the wrong way. I watched a show once where a gastric bypass surgeon said of one of his patients: "This guy told me, 'Doc, how can I be so fat? All I eat is oranges!' What he didn't get is that, if you eat 5000 calories worth of oranges, you're still eating 5000 calories."
