Earlier quoted context omitted.
Existential threat means things can’t get worse (you just stop existing). Ignoring the potential of geoengineering implies you are sure that we have a better solution which will work. I hope we don’t have to use geoengineering but I am certain the status quo of hoping renewables and batteries in the 1st world will save us is not going to work.
Threat is not certainty. If somebody is out there to kill you, you are under an existential threat, until they are caught. It's not a reason to start playing Russian roulette, saying "I'm already in danger, cannot get worse than that!"
I’m not saying this is a good way to think, just that it shows the fault in your logic.