99.9% of those saying that nuclear war might "cause human extinction" are using that as a maximalist emotive phrase. And to declare their moral/political position on nuclear war. Not as an attempt at technically correct description.

One could similarly argue that 3rd degree burns, over nn% of your body, are unlikely to kill you. That technical truth only matters to a tiny number of specialists. For the 99.9%, the sole sane policy is still "do whatever it takes to avoid having 3rd degree burns over a large part of your body".