So in 1950 when US was rising while say the Shah's Iran or some country in south America was about as good as they could get, that made US the developing nation and Bolivia the developed nation?China doesn't see it as propaganda and neither do I. China is not yet fully developed as an economy or country. The financial sector is still immature and one part of the economy that needs more growing. It has a lot of years of growth left to become developed. That's exciting part.
It is not a Japan or an UK who are basically matured and are about as good as it gets.
Like it or not, other people before already took the labels developed and developing to mean something else.
The words you're looking for are more like, advancing or regressing nation.
There is no such as a country maturing, ever. There's only ever increasing tech/science/producitivity. A country stopping does not mean it matured, it means it stagnated or regressed.