When you are holding a tough pose, which requires serious strength, focus, and concentration, it's almost impossible to let your mind go to many complex places. Most of the time, if my mind wanders somewhere and gets caught up in whatever buisness it wants to latch onto and take me out of the present moment, I fall out of the pose, I lose my balance, and I have to start over.
For the girls who are suffering with eating disorders, the majority of them are probably perfectionists, and my guess is, they do not want to be caught dead falling over in front of others, so they focus on the practice. In turn, focusing on the practice, including their breath, the asanas (postures), and the teacher's instruction, they start to reap the benefits. Looking bad falling over probably dissapates and they start to relish in their practice and realize they don't have to be thinking about food every minute of the day. That's just my guess.
Yoga is a form of meditation. It has the ability to make people realize they do not have to be controlled by their thoughts. Instead, they can be fully present with a clear mind, that is empty-- Empty doesn't mean no thinking. Obviously, thinking is good, it's necessary. But yoga can filter out the crap- the negative thoughts we all have floating around in our heads naturally.
This is the main reason why yoga is a beautiful, glorious practice. It's a gateway to freedom.