if you read the GTK dev's original post, the plan outlined there is much more subtle than the summary in this article. The GTK devs plan expects most applications to follow the stable cycle, which is released every 2 years and have stable API/ABI, rather than the 6 monthly unstable cycle.
Third party theme authors and application authors that follow the six monthly cycle is cautioned to do so at their own risk. The six monthly cycle is mostly intended for GNOME applications to dogfood themselves.
What essentially that GTK is doing is to treat 4.0, 4.2, 4.4 releases as some sort of public beta for the LTS version that comes every two years. If you want stable API/ABI, follow the biannual release train.