Depends how you define "developed country". The world bank seems to consider GDP as the main factor, if you disagree with the common metric then you'll have to define how you see it.
China has a lot of wealth on display but overall isn't so well developed if you remember the fact that so many people live there. If China were as developed as the US, we would expect it to be something like 3x the economic activity as the US.
GDP per capita:
USA: 57k
Canada: 42k
UK: 39k
...
Kazakhstan: 8k
China: 8k
Libya: 7k
In addition by some metrics, it could be exploiting even harder to achieve further industrialization. For instance, China is way behind the US in CO2 emissions per capita even today, and if you count total CO2 emissions, the US is still #1. If China had equal CO2 emissions as the US per capita (which might indicate a similar level of industrialization), that would be disastrous.
https://www.ucsusa.org/global-warming/science-and-impacts/sc...