I actually happen to think that the report was too dismissive of more leveraged climate change interventions that I expected could be a lot better than the estimates for Cool Earth (especially efficient angles on scientific research and political activity in the climate space), but the OP is suggesting that the original Cool Earth numbers (which indicate much lower cost-effectiveness than charities recommended by EAs in other areas with more robust data) were overstated, not understated (as the original report would suggest due to regression to the mean and measurement error).
I actually happen to think that the report was too dismissive of more leveraged climate change interventions that I expected could be a lot better than the estimates for Cool Earth (especially efficient angles on scientific research and political activity in the climate space), but the OP is suggesting that the original Cool Earth numbers (which indicate much lower cost-effectiveness than charities recommended by EAs in other areas with more robust data) were overstated, not understated (as the original report would suggest due to regression to the mean and measurement error).