I've not read the book indepth, -- just glanced through it at the library, but what I do note is that it's not easy to define a Celtic physiology as distinct from, say, a Germanic or other Indo-European/Aryan group. Hence, you'd have to base it on linguistics and the evidence does not seem to define that Tocharian was any more related to Celtic than to Iranic-Indic etc.
Victor H Mair (Chinese Language And Literature) makes the Tocharian - Celtic connection...he seems to have the credentials to do so. I'll go with his 'reading' on this subject.
"Throughout the 1990s, Professor Mair organized an interdisciplinary research project on the Bronze Age and Iron Age mummies of Eastern Central Asia. Among other results of his efforts during this period were three documentaries for television (Scientific American, NOVA, and Discovery channel), a major international conference, numerous articles, and The Tarim Mummies: Ancient China and the Mystery of the Earliest Peoples from the West (Thames and Hudson, 2000)."
Let's suppose that they have it backwards.
What if the original people with the red-hair, blue eyed genes originated in Sundaland and after their home-land submerged (Sundaland was about the size of present day India), made their way up the river valleys of Asia, across the steppes/mountains and into Europe 8,000 years or so ago. If the dispersal point was SE Asia, some would have gone searching for a new home land by sea and some by land...and, they would have gone in all directions perhaps explaining the persistant stories of red-headed people everywhere.