Happy to see somone outside of here writing about this study!

As bad as it is, it's still beeing promoted by "Helse Fonna" - the health region where Stubbhaug is. The situation is a bit nightmare'ish :/
I'm not sure how much impact this bit had though:
So on top of what has already been mentioned: "Many patients with CFS/ME tend to be critical to biopsychosocial interventions, and possibly most of these patients did not accept referral to the clinic, contributing to the possible selection bias." I'd say that this was another quite important *issue* that faced the Stubhaug study.
What isn't clear from the study description (or at least I'm not finding it mentioned?) - is that Stubbhaug have made a formal deal with "Helse Fonna". All suspected ME-patients are to be referred to him for diagnostic. Then, if you are found to have ME - or "ME", (his version), it's also him that delivers the introductory classes and/or rehabilitation.
All health regions are mandatory to have diagnostic, classes giving you the basic information and rehabilitation. In Helse Fonna - that's all him.
It's possible to get referred to another health region, but everywhere have waiting lists and they will prioritize their own patients. For classes, if your own health region offer these services, you need their permission to go anywhere else - as they will get the bill. Added to that, NAV = work and pension, will demand that you have been to these classes before applying for disability.
So unless you have the health to travel, and the money to pay privatly for diagnostic - you're pretty much trapped

It's not impossible to go elsewhere, but it's not easy,
Through a cooperation agreement with Helse Fonna, patients with CFS / ME in the admission area are admitted to the clinic for diagnosing and treatment. The clinic also serves as a clinical third-line offering in Helse Vest, and Helse Vest RHF has strengthened the clinic with additional specialist position for clinical research.
https://helse-fonna.no/nyheiter/nytt-behandlingsprogram-gir-hap-til-pasientar-med-kronisk-utmatting