granted it is an inaccessible part of the world, I dont think they were as isolated culturally as you might think, particularly in the 600-1000 CE timeframe when buddhism took over. at that time, tibet was a massive empire, which included the trade arteries through the tarim basin, an important juncture in the silk road and abutting the abbisids, as well as into india. they would have had access to a lot of cultural interchange at that time (otherwise, they'd still be practicing bön...)
https://en.wikipedia.org/wiki/Tibetan_Empire
