Doctors or others who say that a disease is psychosomatic do not have any evidence for that; why not because it doesn't exist.
The mere fact that routine testing is normal is not evidence that something is psychosomatic. But many doctors do not understand or realise this.
In Parkinson or ALS for example, routine testing is also normal.
And in MS even with the advent of the MRI scan, on average it still takes 1 to 3 years to be diagnosed, meaning that these patients according to doctors like Dr O'Sullivan, have a psychosomatic disease, before after 1-3 years they are finally diagnosed with MS by a proper doctor.
By simply checking the cytokine levels you can find that Abnormal cytokine levels distinguish ME/CFS from Gulf War Syndrome and healthy subjects, ( http://www.sciencedirect.com/science/article/pii/S1043466614006024) which clearly shows that ME is a physical disease and not a psychosomatic one.
But many doctors do not know what cytokines are, which are hormonal messengers of the immune system, and you need specialised Laboratories who can do difficult immunological testing for this.
Venus Williams, the successful American professional tennis player, who has won 7 grand slam singles titles, including Wimbledon 5 times, had a psychosomatic disease for 5 years before she saw a proper Dr and was diagnosed with an autoimmune disease (Sjögren's).
A very sporty friend of mine had vague cardiac problems when he was 26, I sent him to a cardiologist, all tests were normal, so the cardiologist said it was psychosomatic, 2 weeks later during a 10 kilometre run he collapsed shortly before the finishline, a doctor who happened to be there tried resuscitation which didn't work and my friend died of a psychosomatic disease.
Did the cardiologist apologize to his family, obviously not.
Every doctor can tell you those sort of stories, including Dr O'Sullivan, but self reflection, just like doing research and not ignoring it, is not one of her good points.
All this and more illustrates that if you don't do the right test, you don't get the right answer.