I thought it was proven to be a “forgery” made by a priest from some vellum he cut out of an old book and given to a friend as a gift who knew what it was. I think the priest was an antiquarian or historian and his friend a collector. I think the priest just wanted to “try his hand” and, considering the ongoing controversy, he exceeded his ambitions.
Maps have secrets. It takes a massive amount of information to draw what appears to be...A Simple Map".
No one has proven anything about it, any more than anyone has proven that Schliemann faked the so-called Mask of Agamemnon, or that Evans faked that cool ivory statuette. One allegation is that part of the drawn image is older than the “map” images.