[asjdflakjsdf]

I don't know if that would be qualified as an unbiased source. The author seems to have an agenda going by the footer...

The fact that he puts the word "Celtic" in quotation marks while suggesting there was no such culture is interesting.

The footer of the site:

Ad Fontes is a quarterly publication of The Davenant Institute.

The Davenant Institute seeks to retrieve the riches of classical Protestantism in order to renew and build up the contemporary church.

[User23]

There is no such thing as “celtic” culture. Celtoi is a word of precision roughly on par with “barbarian.”

[NoboruWataya]

Despite the looseness with which the Romans originally used the term, it seems well established that the groups of people who came to be regarded as the "Celts" did have some cultural traits in common: https://en.wikipedia.org/wiki/Celts

Perhaps it is more accurate to say there were Celtic "cultures" rather than a single culture. Regardless, I don't think the etymology of the word itself is determinative.