From the Wikipedia article it appears he's taking the "nuclear winter" approach. I pointed out the criticisms of that in my comment above. But even his notably extreme version still doesn't support the notion that this would lead to literal human extinction. It also appears he's rather out of date, he hasn't been a nuclear war planner since 1971. If you look at my sources above about humanity's nuclear arsenal over time you'll see how significant that is.

Please keep that at forefront here. This subthread is about human extinction, not about "oh no I guess we have to reinvent steam power" or some other drastically less-bad outcome like that. I have never said that I don't think a nuclear war would be bad, of course it would be bad. My position is that it's not going to render humanity extinct.