Any application that depends on either of those indirectly depends on libpng and dozens of others. Its a dependency tree, just cutting it of after the first dependency does not make sense in any context and even less so in a context that claims to measure package reuse.
The KDE and GNOME platforms contain all of those indirect packages. They contain basically everything generic that you need to build a GUI app. Then when you go off in to the specific little tools you need, you bundle those in.
Exactly. The situation is the same on Windows and macOS: the platform contains the vast majority of dependencies that might be shared across apps. Beyond that, apps bundle whatever they specifically need.
