In that case it wouldn't matter much to an individual making personal decisions about their environment. Either way, an environment with enough outdoor air exchange to keep CO2 low would avoid these effects.

But hypothetically, what other trace contaminants would you expect to be so universally correlated with CO2 in different environments that they could account for repeatedly observing these effects in different studies? That seems implausible.

There's tons of things in the air. Dust (and many different types of dust at different levels, both in contents and size), other gases at the trace level - lack of ventilation not only increases CO2 levels but everything else, too. And then there's the people factor. Who says that people who have proper ventilation and CO2 control are the same as the ones who don't?

If they really want to do a robust study they need to do an intervention study with clear levels of CO2 accurately controlled and the rest of the air being identical for everything else, otherwise it's purely meaningless. (it's doable, by the way).