This is a small point probably not known to most people who haven't had a lot of EDO (eating disorder) treatment experience, but there isn't one way to "look anorexic." A person using anorexic behaviors and experiencing symptoms may be on the way up or down from a low or high weight, just for one example. A lot of weight restoration is invisible because you have to put back on all the weight you may or may not have lost from your internal organs, such as heart muscle. Anorexic people who appear of normal weight can still have many of the same risk factors for death, including abnormal cardiac function.
I think we've seen some evidence that BSR had some noticeable fluctuations in weight throughout her adolescence, but we haven't seen her look clearly emaciated -- and that doesn't mean her eating disorder wasn't or isn't serious or life-threatening. I bet that if she had been nourishing herself adequately throughout the pregnancy, her belly at the time of prom would have been much bigger.
Now did any of that cause the baby to be stillborn? That I don't know (my opinion is no, I think the baby was born small but alive).
ED-NOS is currently often used to label an eating disorder that doesn't fit in the clinical definition of either anorexia or bulimia. This often means a patient is switching back and forth between mostly restricting or mostly purging, which tends to happen a lot with people in weight-restoration focused treatment. It is also commonly used when someone fits all of the criteria for anorexia nervosa except their BMI is higher than the diagnostic threshold (usually in the 16-17 range depending on the other criteria). This is currently mostly a workaround to get insurance companies to pay for treatment even as weight restoration continues, or before patients reach deathly low weights, as statistics for recovery are improved in either situation if treatment can proceed.
Anorexia Nervosa: ICD Criteria
Not only are eating disorders absolutely mental illnesses listed in the DSM-5, anorexia nervosa has the highest mortality rate of any psychological disorder.
Focus on anorexia nervosa: modern psychological treatment and guidelines for the adolescent patient
NIMH » Eating Disorders