True enough, but reason in the 5th century couldn't establish anything about Creation because nothing was known except what was in Genesis. (Pagan theories were obviously out of court.) And since God can do anything that isn't self-contradictory, the Genesis account, seven days, Garden of Eden, Serpent, etc., should have been okay.

Did Augustine specifically write that Genesis was one of those things that was contrary to reason? Did he believe that miracles were contrary to reason? I don't think he did.