Although if you were to ask them to guess at the etymology, you probably would get a lot of disappointing answers.
celts consumed plenty of dairy. kefir is dairy. clotted cream is fermented and buttermilk and cheeses are fermented.
scandinavians eat fermented fish, and there was plenty of exchange and living side by side between scandinavians and northern celts.
Also, cook books are a modern invention. You're not going to find a collection of thousand year old recipes by looking at written records.
[1] https://eatshistory.com/the-5-oldest-recipes-in-the-world/