True, and dangerous; this even happens naturally when volcanos displace oxygen and create lethal stretches of depressed ground. (https://en.wikipedia.org/wiki/Mazuku)

But not the issue here. CO2 is a tiny fraction of the atmosphere, and apocalyptic global warming happens well before O2 displacement becomes an issue. Bear in mind that humans can function effectively up to several thousand meters - the O2 loss from low pressure is vastly larger than any possible CO2 displacement.

If CO2 is going to have physiological effects via the atmosphere, it's based on chronic exposure and high blood concentrations. There are some studies suggesting that causes cognitive harm, and others suggesting it doesn't.