They were not given a biomedical label, nor were they given any credence via testing until many years later.
I hate to disappoint everyone but I have to repeat that 'OI' is not a 'biomedical label'. It is just the latin or greek version of 'not being able to tolerate standing up'. It is not a condition or an explanation. It is therefore neither biomedical nor psychological or anything. It is just a name for a symptom.
Tests do not tell us whether you have OI. They simply suggest vaguely what mechanisms might be causing the OI. The two mechanism types that Rowe refers to are POTS and Neurally Mediated Hypotension.
It is still not clear to me how anyone can tell whether the tachycardia in POTS is a response to a blood distribution change that causes symptoms or a response to the symptoms, which may be caused by something else. POTS is also a very confusing term because it is often used as if it was itself a cause, that might cause other things like brain fog.
Neurally mediated hypotension seems a pretty vague category, except in that the blood pressure must actually go down.
If, as seems from the thread, a lot of people get problems with sitting up, not just standing, I am sceptical that the symptoms actually have anything to do with blood distribution at all because it is pretty unlikely that there is going to be enough of a problem to cause symptoms in a sitting position.
What worries me is that most of my colleagues are not that good at physiology and tend to assume that the most obvious answer is the right one. As an example, virtually all doctors think that swelling of the feet in heart failure is due to raised pressure in foot capillaries and in nephrotic syndrome it is from low albumin levels. However, back in the 1960s Eugene Renkin showed this could not be the case and in fact simple observations on patients show it is not. But because the usual story is easier to understand, or has got in the books it continues to be taught.
I would like to see some real scientific investigation of ME/CFS blood distribution. Maybe David System is the person to do this but he has not published anything as far as I can see. We need a real science of OI rather than a cosy story that sounds 'biomedical' but is actually too vague to be any real use.