We know of the existence, in ancient times to the present, of Celtic languages.

We know of an ancient material culture now called "La-Tene" by scholars. It is assumed that the originators of these ornamental styles spoke Celtic languages. But other speakers of Celtic languages did not use this cultural style.

We know of the eastward expansion, in antiquity, of warbands that were identified by their enemies as "Celts", presumably because their chiefs spoke Celtic languages.

To what extent it is useful to lump these diverse things together under the single heading "the Celts", with a definite article (as if referring to a single homogeneous entity), is not clear. This bit of intellectual bookkeeping might reasonably be dismissed as "rubbish" if it could be shown to be more useless or misleading than otherwise. On the other hand, the concept of "the" Celts, carefully used, might be a useful bit of intellectual bookkeeping that can help librarians and scholars organize and classify the results of their research, in which case dismissing it as "rubbish" would not be warranted.