I suspect the percentage of people scrolling down the page is continuing to increase, but even if it's not, I don't think it's that important. The majority of people visiting a typical Web page don't "convert" anyway (that is, they don't do something that the page is designed to encourage). The key is to give a good experience to those who do want to engage and providing content below the fold is a legitimate way of doing that (and, as Neilsen notes, is better than 'paging.')
All that said, saying "You're still reading. Pretty crazy, huh?" doesn't prove a point though. Clearly the percentage of people who scrolled and are reading.. are reading. But I might just be in that 20%. Without hooking up some JavaScript to your page, we can't get a figure on what percentage that is ;-)