It's not and wasn't meant to be an argument in favor of AGW. It was meant to show that someone arguing against AGW made a big, easily shown and easily correctable error. To make an argument for AGW, you combine observational data with climate models that allow the determination of climate forcing factors, and what emerges from this analysis is the most important climate forcing factors.
Facts work for me. And when it comes to AGW, they're not there.
Feel free to think you're correct.