Pretty cool demo. Are you familiar with Windows’ RC files? I haven’t programmed a Windows GUI in 25 years, but I remember windows and dialogs were created in this declarative language, and I reckon you could in theory extract them from a binary and change the shortcut of a menu or text of a label.
I doubt that’s still possible with modern Windows tech.
Also, GTK GUIs are often described with their own XML format, which ships alongside the app and placed in /usr/share/somewhere. No idea about QT and VLC, but I wouldn’t be surprised to learn their GUI declarative languages are eventually turned into C++ so cannot be updated after the fact.