It's not a "hoax" by any measure — the scientists working on it do so earnestly and do so within the bounds of real science. So we can at least use the right terminology for it. It is a hypothesis, and there have been teams of competent scientists doing real, peer-reviewed work on it since the early 1980s. Is it a valid one or an invalid one?

The resolution comes down to what model are you using for the injecting of smoke and soot into the atmosphere after a large nuclear attack, how long that will stay in the air after the attack, and how they will interact with sunlight. The competing models use different estimates for the various values. With some estimates, there is smoke, but it gets "scrubbed" out of the atmosphere through different mechanisms (like rain) over a short amount of time, and so any climate effect is small and temporary if even perceivable. With others, the smoke/soot get up there and stay up there and reflect a lot of sun and cause fairly wide-spread temperature decreases that can take years or decades to return to the status quo.

Which of these is correct? I don't know. I have met people who work on this and they all seem intellectually genuine to me. I also think that most of the people who work on this (one side or the other) tend to be inclined towards one of these solutions being the right answer before they start the work, and their assumptions of optimism or pessimism in the numbers reflect that. But as far as I can tell we don't have perfect simulations and we don't have perfect understanding of the relevant phenomena, because (fortunately) this sort of thing has not happened before. We do have some data we can feed into it (the combined bombings of WWII — about 4 Mt of explosives spread out over a huge area and several years — did seem to create a small temperature dip globally; the torching of the Kuwaiti oil fields in the 1990s did not seem to have a measurable impact globally), and those can help refine the models.

But it's all models. And you're modeling two difficult things. One is the climate, which we're getting better at, but is still hugely difficult the model. The other is a large-scale nuclear exchange, which requires a lot of assumptions since we don't have a lot of source data for that (thankfully). "How much burning of modern cities would be caused by modern weapons?" is a question that is very hard to answer — you're modeling a lot of effects that we don't have good experimental data for (because, again, we haven't nuked modern cities with modern weapons).

If you want a better understanding of the whole thing, Lawrence Badash's book A Nuclear Winter's Tale, does a very good job of outlining the early history of the work, the various responses to it, and the modern updates to it. Badash is on the side of it being a real phenomena, or at least a phenomena that if it has even a small chance of being real, you should assume is real (because to do otherwise is reckless). But he gives those who object to it their day in court as well. It is written for a non-technical person. It will not resolve the question for you, but it'll help you see the various reasons that different people argue differently about it.

I'm not saying that everybody's opinion is equally valid — but for a non-expert it is not going to be possible to really determine which of these models is inherently better at the moment. The number of unknowns are significant.

I miss it every day when I’m on 76. Was a great break especially in the off season. I was horrible at it but really good at hiding and enjoyed every second of it.