I don’t understand your statement. I can have the windows closed all day with no AC and not worry about rising co2 levels. Why would it be different with this device running and circulating air? It’s not like it emits co2.
This looks like a wall mounted monoblock so you would be right but if it’s a split device with an outside unit, it does actually bring in fresh air. This is wrong, split units dont bring in fresh air either. The only setup that consistently brings in outside air is a monoblock with 2 ducts.
Either way, I’m pretty sure running an AC with the windows open is never good advice even if you can afford it.
The only point of concern would be if you have a gas heater for water etc. in your apartment and run a monoblock AC with just one exhaust hose blowing hot air outside while sucking in new air from the inside. In that case, the negative pressure created by the AC can potentially pull gas that would usually go out the chimney into your apartment.
And how exactly does an AC intensify this issue? Because lets be real it’s not really a problem people usually face in their daily lives. Some people leave their windows closed for days during the winter (Which is also bad because of mold etc. but thats another story). Most places are not nearly well insulated enough for it to be a problem. And if it was a problem, having an AC device or fan that circulates the air and evens out the co2 levels across the house would actually help alleviate it somewhat