I would say we don't really know if it's so called Long COVID. I mean that I have taken care of an ME/CFS sufferer long before COVID was around. So it's a set of diseases grouped by their similar symptoms. Autism and ADHD seem similar in that regard
Modern understanding of autism / ADHD sees the two as part of a spectrum of neurodiverse conditions (think of an audio mixer of many different traits, more than a side-to-side slider), but I don't see the connection with ME/CFS, whether or not it's caused by a covid infection.
ME/CFS has been around for way longer - prior to Covid, the Epstein-Barr virus was strongly suspected to be a trigger for it. Unfortunately ME/CFS was (and in many cases still isn't) widely known among doctors, so diagnoses were hard to come by and treatment even harder.
At least now that there is money flowing into ME/CFS research and knowledge about it gets more widespread, "old time" ME/CFS sufferers have a perspective now...
It already had a name - post-viral fatigue syndrome. It's happened after other viruses before.
How does this compare to the base rate in a similar population? Two thirds sounds like a reasonable estimate for "ever had COVID" in Denmark in 2022, though maybe a smaller percentage would in fact be seropositive. It would be interesting if self-reported long COVID had little or no correlation with having had COVID at all.
A better term may be "Long Virus" since multiple viruses can trigger it, Covid just happens to be the most recent pandemic.
The results would potentially scale to some sunset of ME/CFS suffers neho never had Covid, but it makes sense to start with the group we at least have a common starting point on the cause.
Some long covid symptoms are nothing like ME/CFS such as a persistant loss of taste and smell. And there are people with ME/CFS who never had covid.
Anyways, I am sure that research goes both way, that is, studying the consequences of covid and the causes of ME/CFS. "meet in the middle" can be a good exploration strategy.