The article states that CFS is not psychosomatic and this was a misconception held by physicians previously.
e.g. if you happen to have blood work from a patient pre-CFS and post-CFS it's very, very common to see a raised T-cell count and an immune response against the Epstein-Barr virus (or some other retrovirus) in the post-CFS that isn't there pre-CFS.
Of course, it's not a great diagnostic tool because you need the patient to have had some blood work just before they got sick. But it does make the "it's all just psychosomatic for everyone" argument a bit unlikely.