Its an interesting statement. There's a subset of MS patients who have horrendous fatigue and there are other diseases with high rates of fatigue. I believe that he's suggesting that the behavioral treatments he's evolved for CFS should working days other ' more highly defined' diseases ('real diseases'). This is interesting because it implies there is physiological basis to these types of therapies.
I rather think there is; these therapies focus on reducing the level of stress in the body which should translate I would think to improved immune and HPA axis functioning and hopefully at least somewhat less fatigue.
He's basically saying that the fatigue in chronic fatigue syndrome and the fatigue in these other diseases is similar - which may be a big switch for him.