It's strange that to be diagnosed as a bulimic you don't have to be underweight.
I lost 70lbs, went from overweight to just underweight but I wouldn't say I had an eating disorder til I was afraid of food, obsessing over calories, purging everything I ate and fasting when my gag reflex was ****ed. I knew I had an eating disorder because my happiness was determined by the loss and gain of weight and the smell and sight of food distressed me.
So yeah, I agree with you that weight shouldn't be used to determine if someone has an eating disorder.