I think that’s right: from 300 AD to 1700, nobody used the word. Medieval Latin dominated in the meantime, and so preferred a soft c in Celtae without an academ

I think that’s right: from 300 AD to 1700, nobody used the word. Medieval Latin dominated in the meantime, and so preferred a soft c in Celtae without an academic to insist on the k in Keltoi.