If you read the article, you will see that the cognitive effects of covid-19 are not only for critically ill cases. Figure 3B, specifically, shows cognitive effects of milder covid-19 cases.

According to another study, covid-19 can infect brain tissue specifically: https://www.altex.org/index.php/altex/article/view/1924

That may be why it is particularly dangerous to brains, more so than other viruses.