I always did like your way of thinking. Maybe we've found a group of hybrid Modern Human - Neanderthals, huh?
The people he defines as robust were found in these locations:
* Qawrighul
* Charwighul
* Yanbulaq
and later he says that they're closely related to people known by these names:
* Afanasevo
* Yamna
* Srubna
But not related at all to these people:
* Sredny Stog
And, all of the above are defined as proto-Europeans...some robust, some not.
The late Grover Krantz, best known perhaps for his interest in Bigfoot, discussed a scenario where archaic hunter-gatherers persisted for a long time in Europe, still vanishing for the most part before literacy arrived, with perhaps some straggler pockets of population lasting until the Middle Ages.
So, they weren't Celts, but they liked to wear the latest fashions from Milan?