A .2 sd reduction is just 3 IQ points (varies slightly on which test/scale is used). That seems like a very small decrease to measure when only looking for markers to evaluate their previous IQ. Testing IQ in general is a fickle thing. I could take IQ tests back to back and have it vary by more than 3 points.
https://www.medrxiv.org/content/10.1101/2021.06.11.21258690v...
The pathophysiology of COVID kind of makes sense in terms of cognitive impairments, especially given loss of smell being potentially (often probably?) mediated by olfactory bulb damage. There's a prototype of SARS-CoV-2 infection progression that begins in the nasal cavity and sinuses and progresses downward (this is one reason for a renewed focus on nasal spray vaccines). This sort of process is not uncommon in virally-induced brain injury; the viral infection starts in the head, and progresses into the brain. The olfactory bulb area is a common culprit, as it's close to the nasal cavity/sinuses, and in the frontal area of the brain and close to areas closely linked to "higher level" reasoning and similar cognitive processes.