These sorts of assumptions are the default in the climate change literature. Few agricultural impacts studies account for crop migration or for the prospect that we might introduce new cultivars. I imagine the authors are following the lead of the climate literature there, though it obviously massively overstates the impact of cooling or warming
I also think this highlights a wider problem with the nuclear winter literature. The scholars in the field are very obviously biased. Robock and his collaborators write almost all of the papers but clearly have an agenda. Just take a look at Alan Robock’s website—flashing nuclear bomb gifs and pictures with Fidel Castro.
I followed Robock’s work in solar geoengineering and it was also clearly biased. He claimed that solar geoengineering would knock out the monsoons, but his paper actually showed that solar geo would reduce disruption to the monsoon. Researchers in the field expressed frustration about Robock’s meme, which he spread because he doesn’t like solar geo.
If you look at the nuclear winter literature in the 1980s, all of the scientists say “this is bullshit”. The only thing that changed with the second round of nuclear winter papers starting with Robock was that he used modern climate models. But the criticism was never about the climate model, it was about how much particulate matter would get into the atmosphere and how long it would stay there. So, the idea that modern science validated nuclear winter is just wrong.
A very prominent climate physicist told me that the assumptions in the Robock papers are turned up to maximise damage rather than to actually be plausible, and that people are scared to point this out because of the politicised nature of the debate on nuclear war
The smoke estimates in the Robock papers didn’t change despite a massive decline in the nuclear arsenal.