While the premise from the headline is ridiculous, considering the contribution to CO2 emissions caused by human metabolism is not.

Humans don't have a good efficiency, compared to e.g. a electric motor.
Also, the "fuel" used by humans itself typically already comes with a gigantic CO2 burden.

That's also the reason that biking without electric assistance is not quite as carbon neutral as most people believe.

Putting it this way: The primarily meat-based overconsumption prevalent in many western countries offers many opportunities to significantly reduce CO2.