They probably wouldn't no. However he's talking about humans alive today. Whilst I'm not really keen on this distant ancestral concept due to it essentially being a statistical trick, it's not unreasonable to assume that the descendants of Gauls/etc would have married someone in that descendancy tree, somewhere along the way.

The place this really falls down would probably be extremely isolated tribes which have had little if any external contact - there's very few nowadays, but some are remote enough that no pairing has probably happened.