Would it make a great deal of difference to the underlying ME/CFS? Deterioration could affect people's severity level, meaning that when it comes to participation they have greater access needs than before, but severely ill people who haven't deteriorated due to Covid would be in the same situation.
I don't think it's very clear about the difference mild Covid makes to NK cells, or what proportion of people any changes affect—it seems unlikely anyone would spend a lot of money testing large groups of recovered people to find out. There might be data on immune cell count and function from those who've been hospitalised with severe Covid, but that probably couldn't be extrapolated to people who've had mild illness.