There are all kinds of demos: some of them use models, some of them do not, some are technical feats, and some are artforms. Many are both.

Models are just serialized polygon meshes. We've been using models for demos for way longer than DX/OGL have existed. They're just another tool in the box which you can use (if you want to).

Using DirectX or OpenGL nowadays is like using the CPU: it's just part of the stack. They are probably lower level than you think: using DX/OGL isn't just doing something like drawModel(model, x, y, z), it's way lower level than that.

This demo in particular is not very different from old school demos. Back in the day we had interruptions, now we have API calls, but in the end shaders are just code. Elevated uses D3D to execute them in the GPU but that's all. And the synthesizer is apparently coded in ASM.

Even if you really miss the old school platforms there are still demos produced for them, often pushing the limits of what can be done.

Check http://www.pouet.net/ for lots of impressive demos.