I disagree with you regarding Activities. For apps they do most of the screen management the developer needs just fine, especially if they are backed by a strong view and data model. If you download the App you can actually really quickly make out where they leverage the single Fragment (switch the project category via the drawer) and it makes sense there; but I think the same App build with a single Activity and Fragments would be much messier (especially regarding animations and back-navigation) and they'd gain (next to) nothing.