That’s what NVidia wants you to think. In fact, it only works on NVidia because their drivers let you get away with more than they should.
Unfortunately it won't run on AMD cards due to their buggy drivers.
I'd say that the truth is nearer to somewhere in-between the two.
You're absolutelly correct, it's a bit of both. I don't want to turn this thread into an NVIDIA vs AMD discussion so I'll be brief:
The demo was originally coded an tested on an NVIDIA. Later, I re-tested it on an AMD and concluded that I was doing a few things I shouldn't which NVIDIA drivers let me get away with. After these issues were fixed I expected the demo to run but instead stumbled at a bug on the AMD driver implementation of Shader Buffer Objects. The problem was discussed in this thread at opengl.org.
This is why the demo has no compatibility whatsoever for AMD boards, their support for new OpenGL extensions was so bad at the time that I didn't even bothered applying the corrections I made (to remove weird NVIDIA stuff) to the demo that is available for download. All this was about 1 year ago, AMD has probably released new drivers in the meantime and maybe they have already fixed the problem.