It's not even breathing that's the problem, it's the claim that higher CO2 is great for plants. It can be, however the plants that grow from higher CO2 levels (particularly crop plants) produce their plant mass differently than with less CO2 unless compensated for, like in a controlled greenhouse. Directly because they are getting a different ratio of nutrients and gases.

Add that to all the other factors that threaten food supplies thanks to warming. Someone at some point saw that plants get green at high CO2 levels and thought it would work as an argument against climate change, not understanding the details (or not caring because it suits their purpose).