The problem with this idea is that it does make intuitive sense. There's that whole analogy of, "If you have a steady income, you're more willing to spend some of the money you have in the bank. If you just lost your job and don't know when you'll have another one, you hold on to every cent you've got!"
It's just that the body doesn't actually have many ways it can accomplish that. The body can slightly slow down certain processes, but it's not like it can just say, "OK people, we're going to have to start laying people off. Liver, you're off until next week. Kidneys, the week after that. Heart, you're now part-time."
But the biggest issue here is that they think people with anorexia nervosa are voluntarily seeking help and yet being brushed off. Um... That's sort of the entire reason anorexia is such a big problem: they don't think they're doing anything unhealthy.
I know people who are anorexic (or once were). They will smile and nod and tell you they're eating super great and trying to gain weight, all the while thinking you're either an idiot or would agree if you saw them without clothes or are just unnecessarily worried blah blah blah. They go to incredible lengths to hide it from you.
So when a morbidly obese person eagerly admits that they have a restrictive eating disorder, it's pretty natural to be skeptical. Especially since the primary goal of treatment is restoring and maintaining a healthy weight.
I think these people do have an eating disorder: compulsive and/or emotional overeating. Just like anorexics, they pretend the harmful behavior isn't a problem. Just like anorexics, they seek out forums (proana groups versus HAES groups) where people will agree with and encourage them. Just like anorexics, they lie and deceive to keep up their behavior -- in this case, lying about restrictive eating so that they can get someone to tell them to never ever consider dieting and avoid reducing food intake in any way.