Developer of the game here! It's actually not cell shading. That usually has a threshold with the colors so that it looks more like a comic here.
There's a few things going on: edge detection, screen space gradient, and lack of textures. The lack of textures is what makes it feel like cell shading, but I'm not actually thresholding any of the colors.
Hey, developer of Manifold Garden here. The teaser was released quite a while ago when I changed the name of the game. A lot of the game wasn't finished then, but I needed to get something out, so focused on the architecture.
I also gave a talk at GDC about the level design in the game: https://www.youtube.com/watch?v=ed2zmmcEryw&t It doesn't go over specific puzzles, but it gives you a sense of the kind of thinking you can expect to do in the game.
There's a few things going on: edge detection, screen space gradient, and lack of textures. The lack of textures is what makes it feel like cell shading, but I'm not actually thresholding any of the colors.