With the exception of some QtWidgets on some platforms, ordinary binaries are not linked against a specific protocol library with Qt5. Qt5 provides plugins for different protocols and you can choose them at runtime. Even without Wayland and Mir there are currently many different plugins for eglfs, linuxfb (and/or directfb) and xcb (i.e. the standward plugin for X11 now).

So supporting some window protocol is not a matter of an ordinary desktop program (even commercial ones). KDE on the other hand also delivers a complete desktop experience and has to offer some integration into the window manager as well. So if they say, they support some protocol they speak about KWin (and perhaps plasma etc.), but not, say, Amarok or Okular. While KDE and any other party could technically break Qt's platform abstraction by the introduction of some dependence on a certain protocol (e.g. a "Show all windows" button linked to a certain protocol library), it's unlikely that they would do this.