the way out is not necessarily the way in
This is an excellent question.
I don't think anyone knows, really. I believe the mental illness we call "eating disorder" is largely mechanical, but that disordered eating can be influenced by a lot of things. I think a lot of people do disordered eating without having the mental illness and that people can have the mental illness without currently engaging in disordered eating, if that makes sense. We must stop confusing "Eating Disorder" with "disordered eating."
Disordered eating patterns seem to trigger the mental illness for some people - those with a predisposition. Some people need a LOT of disordered eating/stressors to get tipped over, some are like the fairy tale where the princess is fated or driven to find the spindle no matter what her parents do to prevent it. Once over into the mental illness I think the process is largely biological and neither reachable nor usefully explained by logical or psychological means.
That's why I don't think the psychological arguments have much relevance. If you can become deathly ill from an eating disorder from a single incident or a lifetime of pain then the way out probably isn't the way you got in.
I think it is insulting to those trying to recover from an eating disorder to mistake their anguish and bravery with the garden variety body distress that we (cruelly and wrongly) experience in most of society. It's qualitatively and quantitatively not even comparable, in my opinion. That is why all this discussion of "loving your body" and "size zero models' and "drive for thinness" misses the point. Once you are in, the door you need to use to get out isn't even near the one you stumbled in through.
The REST of us need those messages, surely, but the patient really just needs US to get it and live it so when the patient's brain recovers they live in a world that doesn't make bulimia and anorexia seem normal.