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.

[–] MentalEdge@sopuli.xyz 3 points 4 months ago* (last edited 4 months ago)

Those are still what I would consider "sketches". Just not the two-dimensional kind.

There are a couple more detailed assets, but tubes and boxes laying out locations, interiors and buildings is something you do when still exploring ideas and don't want to lock anything down.

The written material suggests the writers got the furthest into it. But this is very much planning, not production.