I apologise, just re-read your comment. I'm working at the moment (in a pharmacy and just had a patient shouting at me cause the idiot the night before didnt give them their medicines correctly) so probably took it out on you.
But I still think the idea of a green world within 200 years is not going to happen.
Chernobyl is a great case since its one of few ground level explosion sites and as I understand it the animals from the region have been sighted as returning for the past 10-20 years, but Chernobyll, Nagasaki, Hiroshima, etc. are all isolated cases. None of these cases happened when simultaneous explosions took place worldwide, IE no nuclear winter or global climate changing event took place (thats not to say the world wasnt effected by those explosions, having heard of cases of teratogenicity or cancers contracted in Scotland after Chernobyll). Plants cannot survive in radically changed environments, its like if you've ever tried to grow a fruit that doesnt grow in your country, its not going to work. It'd be like taking a coconut palm tree and planting it in a UK backyard outdoors, it's not going to grow, and I'd imagine the climate changes resulting from the nuclear exchange would be significant enough to prevent vegetation from growing back effectively.
In fallout lore you are right in that the lack of oxygen didnt kill off plants as seen in FO1, 2, 3 and NV, but in real life I would imagine that would not be the case.