Exactly what is it about CO2, a plant food, that scares you so much? You do realize that green plants react to increased CO2 percentage with stronger growth?
He was asking you to try to 'steelman', or take seriously the strongest version of, the arguments of your counterparts, rather than being dismissive.
"Plants like CO2" is not a counterargument to "Increased atmoospheric CO2 will have a number of outcomes that are net negative for humanity", so I presume they're asking you to actually think about the argument being made and respond to it, not some other, made up one.
You're the one calling me a "doomer"?