Seems to me that people would have been in that AO prior to and during the ice pile-up, not migrate there after or during.
Why would anyone go up into the mountains and high places *during* an ice age?
But, if people were already established in that area during the better climate, then survived the climate change in place, makes a whole lot more sense to me.
http://www.sci-news.com/archaeology/peopling-tibet-chusang-04514.html
“Among some of the best preserved sites for scientists to study is the Chusang site, which is located on the central plateau 50 miles (80 km) northwest of Lhasa at an elevation of 14,000 feet (4,270 m), near Chusang, a village known for its hydrothermal springs and extensive travertine formations.”
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Hydrothermal springs might provide a clue...that valley may well have been ice free.
The point of the article, nice job missing it, is had there been an ice age accumulation of glaciers, the ground wouldn’t have been visible, but covered with hundreds or thousands of feet of ice.