Actually, it isn't quite that simple. If you have gradual or staged onset PEM isn't likely to manifest itself in the early stages. If a person later expresses PEM, did they not have ME/CFS before then? If there is clear continuity in the illness, then obviously they did. Whether you achieve diagnosis is another matter.
People with AIDS had this same issue in the early days. At first it was thought that only the severely ill people had the disease. It was only after the root cause was found that they realized the many people that appeared mostly healthy were also suffering from the same condition, just in an earlier stage. Now, people with the original condition studied are called AIDS patients and have to have reached a certain stage in the progression for that label, while "HIV-positive" is used for the rest of the PWC.
I suspect this has occurred in other conditions as well. I have known people who weren't diagnosed officially with MS in the earliest stages, although their doctors were suspicious and watching carefully for the biomarker to appear. Sometimes the "atypical" label is attached at that point.
Before the research is complete, it's practically impossible to identify all patients with the condition, especially those with milder forms. For all we know, we all have a genetic condition that has been with us since birth and we've been "ME-positive" all our lives. Perhaps it was only after the illness progressed to a certain point that it was identified as pathological.
I'm not sure there's a lot we can do if patients with the mildest form don't have PEM or other easily identified specific characteristics of the illness... other than push for sound research into the cause of the condition and clear biomarkers.