CO2 is usually a proxy for air quality because outside is usually something consistent like 400 therefore deviation from that figure indicates staleness.
Let's just say, if you look closely at the meta-analyses, all the effect size goes away if you omit one certain author....
If true, SAT scores should systematically vary by hundreds of points depending on which room you take it in. This is not observed.
Besides which, proxy or directly high CO2 means you should ventilate...
having said that, it's possible they omitted what other gasses and %s they used for advantage. there could be some mitigation possible there.