To be fair to them, it's a reader's question, and the answer says that in reality, the CO2 we’re breathing out is part of a natural cycle, by which our bodies convert carbohydrates from CO2-absorbing plants into energy, plus water and CO2. As such, we’re not adding any extra CO2. In contrast, burning fossil fuels like coal releases CO2 which has been locked up for millions of years, producing a net contribution to global warming.
Source: https://www.sciencefocus.com/planet-earth/how-much-does-human-breathing-contribute-to-climate-change
