In the British islands, being Celtic is to a large extent simply proclaiming being not-English. Hence a tendency to lump a mixed set of heritages under one umbrella title. On the Continent, this is perhaps a similar tendency but the differential here is "pre-Roman" rahter than "pre-Anglo-Saxon" - or are they all "pre-Teutonic" and the great Volkwanderung?
The ideas of Centic culture were largely built around the finds of the La Tene culture. Now it has been pointed out that the great cauldron actually shows scenes from Indian mythology, perhaps the Hindus are really Celts?
Joking aside, despite its myriad flaws on closer looks, the overall concept of a common trans-European Celtic culture still has much use, and considerable life in it. Providing it is understood as a convenient shorthand not a prescription.