[flohofwoe]

The code is quite similar to a GPU shader function, and it's running on a "fantasy console" which basically gives you a 320x240 pixels framebuffer to poke into (and the code used to implement the fantasy console isn't counted).

Same reason why doing things like this on old home computers or a DOS PC can also be done in a few hundred bytes of bare metal code, because the hardware implements most of what today is called a "rendering engine" and implemented in software.