You're right, carbon dioxide in large enough doses is no bueno, but even then it's more likely an issue of asphyxiation than anything else, and it's really a matter of scale. Carbon dioxide exist at ~400 ppm and safe at 5,000 ppm over 8 hours, compared to 50 ppm for carbon monoxide[1]. I know which one I'd rather use to substitute some oxygen for.

I was just trying to point out that if they wanted to induce hypoxia, there's better gases to be inhaling.

[1] https://www.indsci.com/en/blog/carbon-monoxide-vs.-carbon-di...