Long story short, the incredible thing is that instead of using a painted skybox, they used vertex color:
The article delves into some detail over the advantages of this system, but I just think this is remarkable, and might just be a great way to create vector-style art too. Anyway, I thought you guys might take inspiration next time to make a skybox.
Sorry to necro but the number of faces you have are far outweighed by the fact that you don’t have to use a texture sheet at runtime. All the colour data is baked into the verts. This kind of stuff is actually really relevant for mobile games these days.