I think their point was that increasing CO2 in atmosphere makes it more difficult to keep indoor levels acceptable. First, if you start from 600pm instead of 3

I think their point was that increasing CO2 in atmosphere makes it more difficult to keep indoor levels acceptable. First, if you start from 600pm instead of 300, you reach 1000 ppm faster. Adding to the insult, it is much more difficult to ventilate the excess CO2 away, if the air you use to ventilate contains in itself already 600ppm instead of 300ppm.