In most standard 3D graphics, the CPU passes a description of the scene as polygons to the GPU, which then does two[1] shader steps - vertex and fragment[2] shading. The vertex shading works at the level of triangle vertices, effectively translating and and transforming the vertices, and then the fragment shader colors in each individual pixel.

So for a standard scene, the CPU tells the GPU: 'Right, we've got a room, with some pillars, and a monster, and a few lights, positioned like this', and then the CPU calculates what that looks like.

What Inigo is doing is that the CPU only knows there are two triangles - a quad covering the scene - so it just tells the GPU to draw a flat rectangle. The vertex shader does nothing but maintain the flat rectangle. However, because the fragment shader can be arbitrary logic, rather than just painting it with a solid color or even a texture, it is running its own simulation that involves drawing an entire scene.

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[1] More these days with Geometry shaders, but that's another topic

[2] Sometimes called a pixel shader, although really that's incorrect - Fragment is a more accurate term