They note that the market is small, but what if it wasn't? Imagine we found a way to convert atmospheric CO2 into literally anything we want, at the cheapest physically possible cost of splitting it into C and O2. Even then, converting a significant percentage of it would take as much energy as we got from burning it in the first place, which is an insane amount. So to make a dent in global warming, a significant percentage of the world's goods must be made with this kind of process. Seems like a tall order, though not impossible (e.g. we could grow lots of trees and make everything from wood).