That is true, the founder did have an alpha version working by the time they did the Show HN, but still, if he had made the video without having anything technically made (but just by visual effects), would it still have been less effective? I doubt it, people know what they want and don't want when watching the video, and it would have been clear to the founder that people wanted it and he could've started building it after people expressed their demand. So in the end, even if he hadn't made the full product, Dropbox would still be where it is today.