If that counts as a derivative work, then there are a lot of car companies out there that can sue each other out of existence for their too-similar modern designs. I see little similarity in the maps other than the Lost Souls thing toward the center.
They don't have a leg to stand on. The two works have overall differences in visual composition as well as specific illustrated content. Somebody's got a stick up there #*^ and want's to make a fuss over nothing. I'm sure the lawyers are going to get paid though. Suckers.