The versioning is what's confusing. Basically what they want to do is a major version every two years, breaking API. And not guarantee it for the intermediate versions they intend to release part of GNOME.

Not sure how this will work out but Gtk+3 hasn't quite stable, deprecating API after two release, and making some replacement only available in the last version. Non-withstanding breaking theme on a regular basis. As an application developer, I can tell THIS is already bad.