The Celtic culture in Brittany is not something that survived from pre-Roman times, it's a later re-import from the British isles.
Stop making up silly, false, fake identities.
I think this is an unnecessarily negative framing. All national and regional identities are mostly socially constructed "imagined communities"[1].
For example, most Americans aren't descended from colonial settlers, but they still identify with the colonists when they read about or discuss the Revolutionary War.
Are they really imagined? Or is this idea mostly a "social contruct" that is not really accepted as reality outside of a few "intellectuals". They seem to be describing their delusions of grandeur. Is it not them that have the problem?
The irony of course is that "Celtic" is a silly word to begin with. There's plenty of circumstantial evidence that the 'Celtic' culture of Gaul was reasonably similar (and quite close in cultural 'ancestry') to Roman culture anyway, making assimilation relatively painless. The British isles may have been a somewhat different case.
That's not really how nations or ethnic groups work, though. The fact that some "genetic heritage" from the Celts may be found today doesn't actually mean that Celtic culture has anything to do with the modern nation-state of France.