Cretins. The Royal Navy routinely allows 3000 ppm in its nuclear submarines, places where, if CO2 were really so dangerous at such concentrations, you would expect a much more strict limit. I understand the USN is similar.
Even then, the issue isn't so much the high CO2 as it is the relative reduction of oxygen. It doesn't have to go much below 20% for mild symptoms, such as headache and drowsiness, to start appearing. If you maintain an oxygen level of around 20.5% to 21%, you could have significantly higher levels of CO2 and experience few issues. Worriers and poorly-educated climate-change activists like to do experiments where they sit in a sealed tent or room, and measure the CO2 level while describing how their executive function is starting to fail because of the rising CO2, at levels as low as 1000ppm (which is less than you'd find in a good greenhouse). They don't measure the oxygen and don't account for the fact that they're consuming it fairly rapidly.