Well obviously I am ignorant. I am trying to project my own sense of extreme loss of control onto people saying they have brain fog. And I am not a doctor.
But its a bit like ME, which is not so prevalent now as it once was. Various people (as reported in the press, and one I new personally) were pretty pissed off to have their ME treated with anti depressants - they had a wonderful new diagnosis, a glorious creation once pejoratively called "Yuppy Flu". And they needed it to be untreatable, or at the very least treated by some special means, not generally available to the population at large.
I think its a bit much to predicate a situation where a virus causes brain fog IN SOME PEOPLE, but not generally. Its actually a self-defining illness that cannot be defined.