This is of course a lie. They declare widely used APIs as "internal interface" and break backwards compatibility on a regular basis even on sub minor version updates.
For people who thought they are smart its of course a fuckery.
The most painfull came from breaking the drawing code: Gtk2 moved to Pango/Cairo, Gtk4 moved to GPU Textures. Gtk5 will now break ageing view models. And i hope Gtk6 will break the single threading of the whole Gui.