If a Mars colony mitigates catastrophic risk (existential / extinction risk?) from climate change, then climate change is not an existential risk to human civilization on earth

This does not follow. One possible (although very unlikely) result of climate change is a much more severe situation resulting in a Venus like situation (although not as high as temp and not as much nasty stuff in the atmosphere). If that happens, Mars will be much easier to survive on than Earth, since with a lot of energy from nuclear power, extremely cold environments are much more hospitable than extremely hot environments. Current models makes such a strong runaway result unlikely, but it is a possibility.