This comes from a long line of shoddy “research” exaggerating potential effects of nuclear war. With MAD in place, like it was for the last 70 years, there’s no need to make shit up, it’d be as bad as it can be. At first, they tried to convince people that NOx generated in fireball would strip atmosphere out of ozone; when proven wrong with experimental evidence (supersonic airliners generate some NOx; their output was big enough that it should have some effect on ozone layer according to their model, but it had none) they pivoted to “nuclear winter”:

Although never openly acknowledged by the multi-disciplinary team who authored the most popular 1980s TTAPS model, in 2011 the American Institute of Physics states that the TTAPS team (named for its participants, who had all previously worked on the phenomenon of dust storms on Mars, or in the area of asteroid impact events: Richard P. Turco, Owen Toon, Thomas P. Ackerman, James B. Pollack and Carl Sagan) announcement of their results in 1983 “was with the explicit aim of promoting international arms control”.[91] However, “the computer models were so simplified, and the data on smoke and other aerosols were still so poor, that the scientists could say nothing for certain”.[91]

When proven wrong again with empirical evidence of oil fires of 1991 Gulf War, they shut up for some time:

When Operation Desert Storm began in January 1991, coinciding with the first few oil fires being lit, Dr. S. Fred Singer and Carl Sagan discussed the possible environmental effects of the Kuwaiti petroleum fires on the ABC News program Nightline. Sagan again argued that some of the effects of the smoke could be similar to the effects of a nuclear winter, with smoke lofting into the stratosphere, beginning around 48,000 feet (15,000 m) above sea level in Kuwait, resulting in global effects. He also argued that he believed the net effects would be very similar to the explosion of the Indonesian volcano Tambora in 1815, which resulted in the year 1816 being known as the “Year Without a Summer”.

The idea of oil well and oil reserve smoke pluming into the stratosphere serving as a main contributor to the soot of a nuclear winter was a central idea of the early climatology papers on the hypothesis; they were considered more of a possible contributor than smoke from cities, as the smoke from oil has a higher ratio of black soot, thus absorbing more sunlight.[93][101]

In a 1992 follow-up, Peter Hobbs and others had observed no appreciable evidence for the nuclear winter team’s predicted massive “self-lofting” effect and the oil-fire smoke clouds contained less soot than the nuclear winter modelling team had assumed.[118]

The atmospheric scientist tasked with studying the atmospheric effect of the Kuwaiti fires by the National Science Foundation, Peter Hobbs, stated that the fires’ modest impact suggested that “some numbers [used to support the Nuclear Winter hypothesis]… were probably a little overblown.”[119]

then came back again hoping that someone would not remember the former and believe them. Even one of authors (Owen B. Toon) is the same, they cite their old papers and use old wrong numbers. This is not somebody trying to figure out how reality works, this is somebody trying to sell you a story. That story tries to make them relevant, but they aren’t anymore, and more importantly they’re wrong

This all is also before noticing that 70s era nuclear arsenal doesn’t even exist anymore, so their predictions lack a plausible starting point in the first place. It’s horseshit start to finish