> In the first year of the COVID-19 pandemic I learned that we can estimate our level of risk by checking the concentration of carbon dioxide in the air, because when infected people breathe out virus aerosols, they also breathe out CO2.

The above is from the author early on. So they go out of their way to point out that it is an estimate and also point out the mechanism that allows it work as an estimate in some conditions. "when infected people breathe out virus aerosols, they also breathe out CO2."

The article does not reenforce this through out the article though and leaves it to the reader to keep in mind.