Instead, it's about the best (defined as cheapest, without taking into account side effects) way to cool the earth in a hurry, without considering the long-term effect of your choice of action. They don't quite spell it out to this degree, but that appears to be their question.
They also never spell out why they think this is an interesting question. I think an interesting question is what should we do about climate change. They've instead phrased a question whose answer has no policy implications on this question. There's also many reasons to think they haven't answered their own question correctly, but on top of running away from their contrarian arguments to say they were only looking at this tiny topic, they've come up with an argument that's completely unimportant.