At a certain point, one must take responsibility for clicking "a scroll bar buddy that walks down the page when you scroll" and then being faced with exactly what it said.
I agree that preferring reduced motion and then visiting a site whose sole purpose is presenting motion is an interesting choice, but I don't think the CSS Working Group or web developers in general are in any position to question it.
> Counter-argument against exempting the demo page
I really don't think that's a counter-argument for exempting the demo. It's an argument against ever implementing this feature on an actual website. Or, an argument for using the prefers-reduced-motion check on an actual website.