I see the point of the question itself. If you're demanding people slash CO2 levels because the current levels are too high, but then are shown to have no idea what the current levels even are, it should be incredibly damaging to your credibility. His followup rational is definitely incomplete, because yes different compounds have different potencies, but the question itself was still worth asking.

For the specific example of CO, your minimum value there is a bit extreme. You can be out in the middle of the Pacific or up the Himalayas and still be breathing 0.5ppm. A healthy adult will exhale about 2ppm of CO from normal respiration. Outdoors in urban areas you will be breathing in up to 3ppm. And commercial CO detectors are legally allowed to class anything under <30ppm as "no CO".

That said an increase of constant CO exposure from 3 -> 10ppm has been linked to a small statistical increase in health issues, but that 10ppm cadre is living in the midst of heavy industry with a host of other health hazards that are hard to completely account for in a simple statistical analysis.