The study looks at the impact on growth of GDP, not GDP as a whole, so it is less than 10% of GDP. It's a measure of slowing down the growth in the economy.
Personally, I think the methodology that the study used was very conservative. I would note that it's based on an older IPCC report, not the most recent one that just came out. If that study were run again, I think you'd find more dire numbers.