Lipstick is the compositor and uses Qt Wayland as a library.
And great, in Qt 5.12 it is still LGPL. In Qt 5.15 it is GPL and not available under the LGPL anymore.
Not sure if there is more stuff that might have been switched in Qt 5.12 already. If not, the request for Qt 5.12 is not that unreasable 
It would be nice if people from Jolla would talk more about what is holding this back. That might help against too much speculation by us as mere mortal users.