A hacker differs from a developer not in the presence of source codes and tools, but in the store of knowledge. If I had the knowledge of a developer, I would become one.
Um, ok lol.... there are a lot of other differences than that, but that is beyond scope.
If you can't place your mind set as a developer though, then it is going to be difficult to explain the process, but I will try.
The tile maps were most likely created with an inhouse tool similar to something like Tiled
Tiled by an artist.
This inhouse tool would export the singular tile map into 2 different Saturn specific tile maps, one for regular part, and one rotated 180 for reversed part.
The developer would just import these maps into the project and work with it.
Now if you want to make the tile maps different, for regular and reverse, you are going to have to go back to the artist and request 2 tile maps for him to create.
This becomes a burden on your artist if he is modifying all of the tile maps even though 1/2 of them has a 99% identical counter part.
Instead, your artist can keep the work simple by separating that 1% part out and giving you a sprite asset, and where it goes on screen.
For you as a developer, this is 1 line of code usually.
DrawSprite(assetname,x,y,color)
or something to this effect depending on your environment.
On top of that, you can make quality checking easier.
You are guaranteeing that anything in the reverse castle is good on the VDP2 layer because it is 1:1 with your regular castle, just reversed.