I think lets put it into perspective why the nuclear winter claim is not realistic. Modern cities in both the US and China are mostly concrete and steel, both materials that aren't flammable to begin with. The total urbanized area of the US 3% of it's area, or ~106000 square miles of buildings which would no doubt NOT be all highrises -> less fuel to burn.
In the 2019-20 Australian bushfires, ~20000 square miles of land was burnt, fuel rich wood and grass fires, it darkened the skies and made it look like the world was about to end. That did not cause a bush fire winter in Australia, let alone the world! Yearly 10s of thousands of square miles of forest burn and release their smoke and carbon into the atmosphere with little permanent effect to show for it. If you actually read the papers those doomsday prediction was based on, they basically assumed that every single city that was hit with a nuclear bomb is made of wood, with 0 fire suppression and all dust get instantly teleported to the stratosphere to block sunlight. Read the soviet and american military reports actually assessing the threat, it really is bad science that got popularized and now nobody want to rebuff it because they don't want to be labeled a nuclear war enthusiast. Even then every time they make a new study on it the effect gets toned down a notch, we went from another ice age to eh it'll be cold for 6 years maybe.