I haven't read that book, but I understand that there were Indo-European speakers in the region of the Cherchen desert. I suppose that the region was more hospitable a couple of thousand years ago.
"I suppose that the region was more hospitable a couple of thousand years ago." They were there 4,000 years ago. 3,000 year old fabrics from the mummies there are just like those at the Celtic sites at Hallstat. There is some speculation that they were the speakers of Tocharian A/B.