Oh, please. Have you read the novel? It's deliberately full of anachronisms, both used by the narrator and by the characters themselves (especially Merlyn, who own a complete set of the Encyclopeadia Britannica and references the as-yet-undiscovered island of Bermuda).
There are plenty of legitimate reasons to criticize this production. Not being sufficiently historically faithful to the actual Middle Ages is not one of them, since it was never intended to be.