Indoor air usually has significantly elevated CO2 levels compared to outdoor air. More ventilation cannot push the levels below the levels of outdoor air, but

Indoor air usually has significantly elevated CO2 levels compared to outdoor air. More ventilation cannot push the levels below the levels of outdoor air, but it can get them close. Their argument that assuming today's already inadequate ventilation systems don't change, more outdoor CO2 = more indoor CO2, pushing it above some arbitrary threshold (which is far above predicted outdoor CO2 levels).