I was about to try and write similar without causing distress to others, but you did it way better! So I've amended and chopped my comment down!
There is lots of misunderstanding still about M.E. amongst doctors and scientists.
I used to have a GP, now retired, whose personal opinion off the record was that Fibromyalgia, CFS and ME are all the same disease with a differentiated emphasis on symptoms. That was her opinion and the private opinion of some other doctors she said, although she said some doctors simply don't believe it exists full stop and she told me to start growing a think skin, because I was going to be confronted with much scepticism thinking from professionals.
I have personally also been told by a consultant that he didn't believe in anything except Myalgic Encephalomyelitis. Fibromyalgia and chronic fatigue syndrome were, in his opinion and his own words to me: 'doctors not doing their jobs properly' His words not mine.
The problem is CFS has always had a more fluid origin than M.E., which was said to come on very suddenly and is actually classified as a central nervous system disorder. CFS was said to come on gradually and have any number of origins; post viral, accident, trauma, autoimmune disease, pituitary gland dysfunction. Hence the potential to get better once identifying a possible cause is greater.
I have also heard stories similar to some poster's here, people using certain apps or NLP or certain therapies and they are 'cured'. If you think of chronic fatigue as a cluster of symptoms with fatigue being the all encompassing one rather than as a disease (I neither agree or disagree) then it's less hard to understand how one person can appear to do better than others and that some people will never find the cause, and even if they find the cause, may still not be able to improve enough to recover, whilst another person finds their way and appears to recover.
So you can have a group of people with CFS who recover - end result is improvement in quality of life
Another group with CFS who do not recover/get worse/recover partially - end result is a poorer/severely poor quality of life
A group with Myalgic Encephalomyelitis who may have sporadic periods of doing better, but never recover and can get sick to the point of death - end result can be a very greatly deteriorated quality of life
The middle and and especially the latter group particularly, is vulnerable to any suggestions by well meaning people, but especially people of status and authority that there lies simple cures that simply involve rethinking, reframing stress etc. The DWP actually spent money trying to debunk the condition existed at all to save money and if you refused to do graded exercise (a therapy they are still selling just calling it by a different name) then that was proof that you had a psychological illness and liked being ill and you put in the same category as 'drug seekers' - that's how vulnerable this group is.
In regards to CFS there is mainstream medical speculation about various causes including circulation issues, and small cell neuropathy. I am not up to date on any current theories.
PEM needs specific outlining. Most everyone will feel more tired after activity if they are already running on low energy reserves after all. The difference is how extreme it is. Besides becoming bed bound and in some cases, unable to eat, not being able to even talk, reduced to stuttering and slurring my words like an alcoholic who has been drinking for days or (apologies) someone with a brain injury is the result of doing something as simple as a trip to the cinema on the previous day. Someone said something about Audiobook and I had to smile. It's too hard (and painful) to concentrate on something like that without constant rewinding to the point of not bothering anymore on a bad day. Although hey people may be different.