Thank you for your patience. I looked at the Rodeheffer study which does indeed conclude that CO2 has no effect on cognitive function (among submariners). I have updated my beliefs accordingly.

The nature paper does suggest that it's concerning so I have not ruled out the option of high CO2 exposure negatively affecting humans. I'll have to look into this more at some point.

Your comments have suggested you are, so I'm assuming with low confidence you are biased towards these things being positive, or much less negative than the scientific community thinks.