It makes a lot of sense actually. You get severe symptoms when CO2 makes up only a couple % of the air. And can become fatal at like 5%. There’s not like a hard line where you suddenly die, it’s a gradual thing. It very much makes sense that we’d notice minor symptoms at a few thousand PPM when it only takes like ten thousand to feel it severely.

1% (10,000 ppm) is enough for the person to become aware something is odd through drowsiness or an elevated heart rate.

I don't think it's too far-fetched for a quarter of that to cause subconscious cognitive effects, that could be measured in tests.

I got a monitor as we had an old apartment with bad ventilation. When I started feeling it I would check and it was always around 1200ppm and would open a window for a bit. Outside air is around 420ppm, but that's not the problem, enclosed and badly ventilated rooms are if you spend a few hours in there.