This is not worthy of an existential crisis. It's worthy of concern, but not a ZOMG freakout. You have to keep in mind that "IQ" is not a robust metric. Lack of sleep blows big holes in performance on standardized tests. Distraction is worse. Any sort of infection can lower IQ. COVID-19 infection does cause measurable gray matter reduction. But then, so does drinking. I'm not a fan of gray matter reduction, but most of these studies pretty much say debilitating disease is debilitating Which should hardly be surprising. Now - is that debilitation permanent? Transitory? Profound? Trivial? Well again, it's a "novel" coronavirus which doesn't mean "exciting and fun" it means "new under the sun" and we're still deep in the fog of war. Get vaccinated. Helps.
When I was on chemo, the drugs would keep me awake for 3 days at a time. I lost the ability to make sense of numbers, alarming for me as a mathematician who can remember tons of numbers because each is like the face of an old friend. I sometimes lost the ability to make sense of time, I could not remember if something happened a day before, a week before, or months, and I could not sequence past events. steve made a short video about that strange experience. After treatment, I slept terribly for a year and my thinking and memory were always foggy. Lack of sleep make a huge impact on my intelligence during that period (however I was extremely production and creative during that time as well. Weird.) I had resigned myself to my condition as my new normal, but my new new normal came around to be my old normal.