Hi, great that you like the game!
For the panoramas SecretCDev rendered scenes in blender using equirectangular camera projections. We then used these images in Godot on a simple cube geometry (it actually doesn't matter as long as it covers everything around the camera) and I wrote a very simple shader (https://pastebin.com/MfvdRgMX) that just computes the view direction and transformes it to the equirectangular projection for color lookup.
At the beginning we actually used the WorldEnvironment for it which also works pretty much the same way, but yes it might be a bit overkill for this scenario, especially if you want to easily edit, e.g. I often forgot to make the WorldEnvironment and the contents unique and overwrote other level contents. Due to this we created the custom system.