The BPS people who declared ME was now CFS last century and brought in the idea of deconditioning, CBT and GET also declared that there was one spectrum of illness which went from being more tired than you wanted to be to bedbound and tube fed in a darkened room. There was only one illness whose defining symptom is fatigue.
That is justification for doing work on the least affected; if the treatment works for them then it works for everyone. They have now broadened this out to include all the so called MUS as a single illness. They do not believe any of it is a disease, which is what they call damage to the body.
When you understand that, it makes more sense of what they do. That is why they "disappeared" ME; it would be like calling the caterpillar and the butterfly two different species.
Too bad for us, it works for them