On re-reading what the link had to say, I have to say you are correct. Augustine did not believe in an old earth, though he did treat the text in an allegorical manner, with his instantaneous view.
I find however, the moment someone treats the text in an allegorical or non-literal manner. What is to stop them from going to the other extreme from what Augustine did?
Someone might have some misgivings about accepting a literal interpretation, but by accepting an allegorical or a non-literal approach, it causes more problems than the literal approach.
Namely, it causes one to assume things from their own imaginations, or perhaps go outside (the scientific community) of Scripture itself for an interpretation, thus denying the sufficiency of Scripture.