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.