It is true that even a limited nuclear exchange would likely destory the Ozone layer and wipe out all life on earth. [regardless of nuclear winter] but that research is much more modern. Fallout is based in 1950's science fiction and a desert wasteland is what they imagined as the outcome of Nuclear war.
I'm pretty sure they knew about the ozone layer and thought that it might be destroyed by nuclear weaponry, I'll look it up.
Andperhaps they believed that nuclear winter would block out the sun for long enough that they ozone layer could reform a bit before the dust clears.
EDIT/UPDATE- I looked it up, we discovered that the ozone layer existed in the late 1800s so they might've thought that it would be destroyed by nuclear weaponry, I don't know and nothing comes up when I Google it...