It's a question of concentration and amount. The article doesn't really say how the marginal increase in CO2 quantitatively affects the transmission. Is it a small increase, is it double? How does that change the virus rates.
"Thus, by purifying the air in a room, specifically through lowering the total acid vapour content (e.g. CO2 , HNO3 ), the viral infectivity can be reduced four times faster by the clean air alone than by sunlight."