I understand the theory. Increased heat from CO2 will lead to more H2O in the atmosphere as well. Models calling for significant heat increase will assume significant water vapor increase, which will further increase heat and water vapor and on and on.
Unfortunately, this does nothing to address a scenario where the models had exaggerated vapor-inducing heat in the first place. Recent temperature data seems to indicate that is happening.
And none of this is an adequate argument against any particular theory. The earth is not a laboratory. Many theories, even outside of this debate, could be completely sound science and yet utterly unprovable or even irrelevant when plugged into a global climate.