DNA is not culture and culture is not genetic. The most likely source is movement across the Pamir mountains from the Greco-Bactrian kingdom. There is documented contact in the 220 BC between Han dynasty China. The Han were immediate successors to the short lived Qinn dynasty. Before there was official contact there would have been movement along the trade routes. Just becasue there is Greek DNA found in China that doesn't mean someone born in Greece was in China. The odds are that its a descendant of one Alexanders soldiers that took a Bactrian wife 100 years before hand.