[def_true_false]

Maybe the author just mangled the quote? It sounds like the context was preventing COVID transmission, it even mentions masks. One can reasonably say that the CO2 sensor doesn't distinguish air breathed out by others (dirty) from air breathed out by others and then cleaned by an air purifier (clean, but higher in CO2 than the outdoor baseline). So, if you are in an urban setting where the expected particulate levels are non-zero, you can tell whether the air around you was cleaned by a purifier using a particulate sensor. If it shows zero, it was probably cleaned to some extent.