I don't understand what that diagnosis is supposed to express. You're starving yourself to death, but you either started at a higher weight, have a lower metabolism or your body reacts differently to starvation, so let's give you a different diagnosis.
It's like saying that people who suffer from depression and don't like pizza should have a different diagnosis. Eating disorders are so unrelated to weight. Eating disorders can cause weight loss, gain, maintenance, or changes in body composition which may or may not accompany weight change.
Secondly, anorexia sub-type binge/purge is a great addition to the DSM. However, AN sub b/p is not a synonym for someone who suffers from bulimia that happens to be underweight.
Personally, my bulimia causes weight fluctuations. My set point weight is 5lbs above being under-weight. Which means that if my weight drops for a bit, I'm underweight. This doesn't mean that I've become an anorexic. When I go from 108 to 107.8 pounds I don't become anorexic overnight. I still have the same mental illness. I still suffer the same way.
I think that the removal of atypical anorexia would really solve this problem. All weight criteria needs to be removed. Patients should be diagnosed based on their mental state, not their weight.
If anorexia stopped being associated with low weight, those who suffer from anorexia and happen to not be underweight could still get accurately diagnosed and treated, and those who suffer from bulimia (or BED, or OSFED) and who happen to be underweight could also get accurately diagnosed and treated.
We need changes to be made, and they need to made quickly, before more patients are inaccurately treated. People are dying. The least we could do is try and accurately treat everyone.