The study really doesn't suggest that. All it suggests is that EBV seropositive people are much more likely to develop MS than the seronegative minor population.
As for long COVID, the data quality is very variable. I'd like to see a controlled study that compares recovery from COVID vs. other respiratory viruses.
It's pretty clear that the causal explanation is the only reasonable one here. (Besides, at least for covid, not everyone who contracts the virus develops antibodies for it: https://journals.asm.org/doi/10.1128/Spectrum.00904-21 )
There's a reason I quoted conservative estimates of long covid. The 2% is from this ONS study: https://www.ons.gov.uk/peoplepopulationandcommunity/healthan.... The high end of estimates is 10-30%, which would mean the end of our current era of civilization.
As for long COVID, the data quality is very variable. I'd like to see a controlled study that compares recovery from COVID vs. other respiratory viruses.