A classic example is Snapchat and Instagram. Snapchat was never something that stood on its own, as everything that made it unique would easily be duplicatable by Instagram, down to the tiniest feature. Theoretically, the network of users would be the unique value but it was nowhere near Facebook's size, and thus would not be able to truly compete. All it took was Instagram copying most every good feature to dominate the space.
In the article's case, each of the apps (Clue, Duet, DoApp, Voxer) were creating something that, while valuable to users at the time, was simply an addition to the existing functionality and in no way was a valuable product outside the context of the phone or iOS itself. Effectively, they were features wrapped in an app that in no way stood on their own.
If you want to make a successful app in the App Store, make something with value to a user that can stand alone.