You are correct after all. I setup some equations and solved for the interior steady state CO2 levels as a rough approximation the steady state interior CO2 lev

You are correct after all. I setup some equations and solved for the interior steady state CO2 levels as a rough approximation the steady state interior CO2 levels are proportional to CO2_outside + generation rate / ventilation rate. Therefore, steady state interior levels will only rise by approximately the same amount as the outside levels, which does seem insignificant.