There are probably many discoveries thought to be more modern, that were actually made earlier than we realise but were never written down. Word of mouth stories are often lost during mass population upheavals like war and famine.
(I linked to where he's getting dismissed as a liar centuries later, but the main story about the voyage is way more interesting.)
Related literature: Chrisopher B. Krebs, "A Most Dangerous Book. Tacitus' Germania from Roman Empire to the Third Reich", W.W. Norton & Company, NY, 2011.
Oh wait . . .