They are C++ already. If they deigned the application well, a UI toolkit change should not be too bad. Not trivial but manageable.
I always hate to see apps move off GTK but their first point is about cross-platform and there is just no denying that Qt has a vastly better cross-platform story than GTK does.
That likely has more to do with the culture of KDE vs GNOME. You can make pretty much the same UI with either toolkit.