As far as I have been seeing/hearing, it's less 'they're not Celts' and more 'they were Celts by cultural assimilation rather than by fire and sword'. Thing is, 'Celt' is so ill-defined that there is quite a bit of wiggle-room there - were the Gaels/Goidels Celts? They had a helluvalot of Iberian influence, after all. What about the Picts? Hell, some historians think that the Sweboz were Celtic tribes as well (though I don't agree with that interpretation personally).