The problem is, Mac OS (I don't know about others) don't have a static scrollbar anymore. There's no way of knowing. The people that make medium.com surely must know that in Mac OS there will be no indication that there's any content to read, and that if they fill the screen with a large image without showing any content, they should at least say "there's some content below".
(Granted you can enable it in the System Preferences, but that's not standard)
(Granted you can enable it in the System Preferences, but that's not standard)