I am joking, but I can assure you that even permanent markers can easily be cleaned off Chessex maps after months if you use isopropyl alcohol to clean it off.
I have a mat that I stored for a little more than a year and everything came off nicely, as if it was never marked in the first place, and it had drawings with both permanent markers and sharpies on it.
If that doesn't work, it means your mat absorbed some of the pigment. Then you can clean it afterwards using acetone, which will break the pigment apart and allow you to clean it off with the alcohol. Dip some alcohol, rub a little, dip some acetone, rub a little more, then dip some alcohol and rub some more. Keep repeating until all the pigment is gone. Just don’t rub it too hard, to not damage your mat.