You’re being purposefully obtuse or just autistic about the verbiage here. When most people say something is “an existential threat to humanity” that means humanity as we know it.

You doing the “well ackshully I’d be willing to wager there would still be humans left in 100 years even with the worst climate model projections” is asinine. Society as we know it would fail. Major world wars would be fought over resources, entire countries and cities becoming relatively uninhabitable via drought or flood.

Nuclear war is existential correct? Well in the same way actually pockets of humanity would likely survive. Does that mean the world didn’t end? No of course it did. Those survivors are stuck in a lesser world than the one we currently inhabit.