Also from the article:
Their model used 100 Hiroshima-size bombs (less than 0.03% of the explosive yield of the current global nuclear arsenal), detonated on cities in close proximity. Because of the closeness, and the effect of the sun on the black smoke particles, enough would rise up to cause a mini nuclear winter lasting about decade.
The proximity of the bombs in time and place matters significantly. Even if you are skeptical of this, you'd have to be not updating enough based on expert opinions to view this as less than 10% likely.
Your comparison between the problems from greenhouse gasses and the particulate matter from a nuclear war is absurd.
Their model used 100 Hiroshima-size bombs
2 Mt. 1.5% of the Tzar bomb, which exploded at Novaya Zemlya one day. Nothing much.
Enough to make as much fire as in Black Thursday bush fire? Which didn't caused a decade long "nuclear winter"?
Just a reminder that some of the old threats are still around (and hence that AI is not only something that can go hideously badly, but also some thing that could help us with the other existential risks as well):
http://blog.practicalethics.ox.ac.uk/2012/03/old-threats-never-die-they-fade-away-from-our-minds-nuclear-winter/
EDIT: as should have been made clear in that post (but wasn't!), the existential risks doesn't come from the full fledged nuclear winter directly, but from the collapse of human society and fragmentation of the species into small, vulnerable subgroups, with no guarantee that they'd survive or ever climb back to a technological society.