This is a Javascript demo. So let's not discount the browser, Javascript engine, JIT, 3d acceleration libs, sound libs, and everything used from the OS that is involved here. Still it is a great effort.
Your argument fits better to a machine like the C64, where demos are really going directly to the hardware and the OS is basically inactive.
Well most of the things you named are inactive. It's using a very tiny bit of external 3d code, and external decompression, but almost everything else out of browser, javascript, libs, and OS is passthrough. The javascript engine exists but does not serve to make it smaller; the code is simple loops and subroutines that would probably be better as native code.
Your argument fits better to a machine like the C64, where demos are really going directly to the hardware and the OS is basically inactive.