The story here is likely off on the wrong foot from the start: our Halloween isn't really meaningfully connected to Samhain. https://adfontesjournal.com/archives/halloween-its-creation-...
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.
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.
https://www.newliturgicalmovement.org/2017/11/the-origin-of-...