No, it's not true. The fact that people like to believe such things without evidence (and are willing to take a KGB officer's assertions as sufficient truth) should give you a hint as to how politicized the topic is, but the matter was extensively developed independently by Western scientists of the highest calibre, and continues to be researched today. There are many uncertainties in the nuclear winter hypothesis, to be sure, and plenty of room for technical discussions and debates, but the idea that it is a "fabrication," much less an external one, doesn't hold up to any scrutiny at all. That isn't how science works.

For a very in-depth history of the development, controversies surrounding, etc., of nuclear winter by a professional historian of science, see Lawrence Badash, A Nuclear Winter's Tale: Science and Politics in the 1980s (MIT Press, 2009). Independent research continues to take place on the topic, using vastly larger datasets and more sophisticated models than were available in the early 1980s.