I do not know of a single case of someone with ME/CFS having major intellectual impairment
People with ME share core diagnostic requirements. Things like unrefreshing sleep, PEM, OI, etc. They all have these factors to varying degrees. Same is true for things like balance or pain or exhaustion. Some people are in so much pain they can think of little else. Some people are so exhausted they cant' get out of bed. Some people need walking aids because their balance would cause them to fall without it,
For some people, their intellect is hit hardest. Brain fog, although such a descriptor does it little justice. The degree of this impairment has been challenging to demonstrate because neurocognitive evaluations are blunt and not really targeted at our deficits. That's made even more difficult without premorbid scores to act as baselines for comparison purposes.
But some of us do have those scores, and some of us have had their intellect assaulted enough to have doctors repeatedly request a battery of neurocog tests. I've been sick for over a quarter of a century and i've felt my cognitive skills slowly erode. That sensation has been reflected in my tests. Since the year 2000 I've had five comprehensive neurocognitive tests and i've seen my IQ drop anywhere from 15 to 20 or more points. You are correct in that I'm not yet peeing in the corner of my bedroom, but I can assure you I feel my Stupid and it is an intellectual impairment.
More importantly, I am not alone. There are many like me. We don't engage as much because we are prone to mistakes, but some like myself try to participate irrespective of our deficits.
pwME who labor under the brain fog slight are a very real component of our community and it would be a mistake to not factor them into whatever algorithm you're wielding to characterize us.