It should be noted though, that the effects of low oxygen are already very well known and well understood, thanks to decades of research funded by air and space agencies.
It should therefore be pretty easy for CO2 researchers to control the outcomes of their experiments against the known physiological effects of hypoxia, to determine what effects are actually caused by CO2.
And aside from that, the dosages being discussed here are far, far lower than would be a noticeable dent in the amount of oxygen in the atmosphere. 1000 ppm CO2, the lower bound of observed cognitive effects, is 0.1% CO2, way less than the point where hypoxia is known to cause problems. The atmosphere is 20.9% oxygen at sea level and while I am not an expert in the math of partial pressure, a decrease of atmospheric oxygen to 20.8% doesn't strike me as sufficient to explain the observed effects. That's a 0.4% decrease in the partial pressure of oxygen, or about the equivalent of a 300 foot increase in altitude, or taking an elevator to the 20th floor of a building, nowhere near enough increase in altitude to make you light-headed.