Additionally, current OSes make it really hard, if not impossible, to get down to this level of bare hardware. There's many layers on top that you're forced to go through, closed graphics command stream, closed graphics driver, closed kernel, closed userspace libraries, and the hardware is much more complex. Really DirectX does serve as the lowest level standard way on Windows to access graphics card hardware.
Nevertheless, I do think that the demo scene is impressive - for a different reason.
I believe this is not about the absolute code size, but the relative code size: Within each demo category, all demos run on almost the same hardware architecture and same software platform. Yet, some of them manage to create effects in 4k which most other programmers wouldn't manage to create in 64k.
It is not about where the limits are (4k, 64k, 32 bytes, ...), but that there are limits at all. This inspires creativity, pushing the border of what people believe is doable within these limits.
Do you really think those DirectX libraries have music players, graphics assets (so that you don't need to procedurally generate all) and other components that intros/demos need?
Writing 4k demos is anything but trivial. It's in no way comparable to video player situation.
But all developers still have the same size limit and access to the same apis. Which means that creative competition is totally possible.
But sure, comparing between 4k demos is possible, but as DirectX evolves the comparison becomes unfair, since the system libraries add more functionalities over time.