I think there's a flaw in your logic here: the fact that a drug that is targeting the immune system helps so much does not invalidate a psychological component in the etiology. It could be upstream (psychological stress could trigger the immune response that leads to CFS), it could be downstream (CFS leads to mental anguish given a patient's deviation from normative life-course, perceived isolation, and the historically-contested nature of the malady), or it could be both.

That said, a psychological / psychosomatic component wouldn't undermine its legitimacy in any way. Thankfully, I think we are entering an era in which psychological aspects of ailments are recognized as critical, beyond the control of the patient (e.g. not reflecting a lack of virtue or wherewithal on their part), and treatable.