Getting exhausted from flying is not due to CO2, because a plane has more ventilation (measured in air changes per hour) than the majority of buildings.
I thought it was also due to micro movements of your body trying to remain stable. Kind of like driving a truck on a rocky road but at a much smaller scale for much longer.
I've not seen that claim before but it certainly seems plausible. It does seem less tiring if you're sitting next to an empty seat rather than a person.