In the 15th Century the printing press hadn't been invented. Prior to Gutenberg's invention, manuscripts were one off, and to get a copy you either had to hand copy it, or pay someone to do so. Maps, such as those, were copied from older sources as well. While those copies date to the 15th Century, they are copies of older maps.
Those older maps sure don't date back 34Mya. So, you're absolutely right that someone clearly has obtained information regarding Antarctica as uncovered by ice, but since the maps we have don't have source data, we don't know who or what they copied.
I suspect that these maps are lucky survivors of information that was once in the Library of Alexandria. Maybe the Vatican archives could shed some light on this void of knowledge. We should find out.