The main algorithm fits easily in 20 lines. However, the JavaScript code is optimized and contains also the initializing routines and input device logic.
It is also a little bit unfair to compare this with shadertoy. He doesn't have to include the render algorithm but relies on a GPU. The Voxel Space engine just draws vertical lines and doesn't need a GPU.
If I extract the code and delete what I can see might be extraneous setup boilerplate (event handling, camera navigation, init function, image download helper utility, various config object initialisations), it still comes in at around 150-200 lines. The render function alone is 38 lines, not including the external dependencies of that function (DrawVerticalLine, screen, camera).
Again, not a criticism of the work, just querying the title.