I agree it's curious they didn't retest individuals at followup, although it's much easier to ask if they contracted COVID than to have them retest. However, t

I agree it's curious they didn't retest individuals at followup, although it's much easier to ask if they contracted COVID than to have them retest. However, they did determine those individuals who contracted COVID were not different in their premorbid test performance from those who did not later contract COVID. This is a big part of what a "complete" longitudinal design would get you. Let's say you did get that test data on follow-up. If post-COVID people who were infected were different cognitively, and not different premorbidly, that would suggest COVID was involved. If they weren't different cognitively, you'd have to explain why the first wave of individuals were different based on COVID history but not the later wave. Still important to show but maybe a different set of explanatory challenges.