"sufferers of other somatic or mental health conditions." I do have some concern over this, though for other reasons than discussed so far. They could have just said "other health conditions" and leave it at that. Instead they used "somatic" which is easily misinterpreted, and "mental" which does include somatoform disorders. Its clumsy language. I am however more sensitized to such language as I am looking for examples of this kind of thing.
Yes, and I think there are ways to get the same point across without using the "S" word - like physical or physiological. "Somatic", while denoting the same thing as "physical" or "physiological", it's seems to be more favored by psychologists. It also might be used somewhat as a code word, selected for its connotations of closely related concepts like "somatization" or "psychosomatic", thereby alerting the audience to their views without being so brazen as to label ME/CFS psychosomatic.
It's also important to note that all of the researchers are from psychological or psychiatric departments at their institutions. They almost certainly intended to study CFS as a psychological disorder, barely even giving lip service to any comparison to physiological diseases.