My assumption would be that it's a natural concept that some or many people would tend to arrive at by trial and error on their own, and so it isn't surprising to me if multiple people are credited as the first, because they all might have been the first in their areas or in their networks. I would doubt it has a single point of origin as applied to this illness. But I could be missing something.
In the early 80s ME research had a resurgence with the likes of Darrel Ho Yen, Dr Eleanor Bell and the Behans showing it was a disease that did not just happen in epidemics. ME was described rather than defined which I think was the usual at the time.
When the BPS people suddenly took over, the gentle days of people talking to each other with an assurance we all had the same disease were over and we were at war. Things may have been going on in the background but that was how I felt about my local group and the ME Association.
There had been a general agreement that the best way to cope with the disease was to rest as necessary and never push yourself if you could avoid it. Tasks should be broken down, rest before and after anything you had to do, that sort of thing. It was not a treatment just common sense advice, though it was a revelation to me. When it needed a name it was called "pacing yourself".
When it was taken over by the BPS and suddenly had formal rules and goals it was horrifying and not at all what the concept was as used by people with ME.
The problem with naming symptoms and other concepts continues to this day and I am not at all sure it is not deliberate. At the very least, patients try to put their experience into words, the BPS adopt them and use them to mean something different which usually trivialises the original meaning or takes away any sense of uniqueness all to back up their thesis that ME is just a dot on a fatigue scale.
PEM annoys me the most. It is a term we use to mean something that defines ME because it does not happen in any other disease. Like the spots of chicken pox make it identifiable, so does the exertion problems in ME. Instead of putting enough effort into finding what is happening, refining what it is and trying to find a treatment there are lots of papers saying it is found in this disease and that disease so the term and thus the symptom is being diluted.
If all spots were defined as marks on the skin it would have become impossible to distinguish chicken pox and meningitis so the disease organisms would not have been found.