Keiferski that link you referenced is relating to the interview stage.
I think the purpose of the application demo is to provide a visualization of what you wrote in the YC application. They say they only view demos from the most promising submissions, so if they're viewing it you already piqued their interest. Short of a live working prototype, I'd create a page that can most effectively show 1) how your app will generate revenue, 2) key differentiators of your app from similar ones. Regardless of the demo format, their time per submission is minimal so be clear and concise.
I think the purpose of the application demo is to provide a visualization of what you wrote in the YC application. They say they only view demos from the most promising submissions, so if they're viewing it you already piqued their interest. Short of a live working prototype, I'd create a page that can most effectively show 1) how your app will generate revenue, 2) key differentiators of your app from similar ones. Regardless of the demo format, their time per submission is minimal so be clear and concise.