As per the link, it was spoken around 500 AD. Which is at the tail end of the Ancient World but it's still far older than Germany as a country.
> The status of Old High German as the origin language of modern German is part of the construction of German history in exactly the manner I have described
Are you just rejecting the entire field of linguistics? Languages absolutely do have descendants, and while there is admixture and external influences there is broad continuity between old German and contemporary German.