Exactly. CO2 is an imperfect proxy, but it's a pretty decent one. And, even for spaces that have high quality air filtration (e.g. a strong commercial central air system with good HEPA/MERV filters), the CO2 measurement can also tell you if the system is configured with a good recirculated to outdoor air ratio (residential systems usually have no such thing, unfortunately).
But yeah, as you said, the best aspect of a CO2 monitor is that it gives you a useful upper bound for the general respiratory virus risk in a space.
But yeah, as you said, the best aspect of a CO2 monitor is that it gives you a useful upper bound for the general respiratory virus risk in a space.