The title suggests that there are files for an actual expansion, half-finished and unpublished.

Because that's exactly what this is. Click through to the source linked in the article, which is a 4chan thread. It's quite a bit more than just concept art, there are actual world map files that were extracted. The files, themselves, aren't made available, but the OP of the thread posted several screenshots of these assets rendered in-engine with a dev build of the game. There appears to have been quite a significant amount of development that was put into this unused area of the game.