These labels aren't useful for extremely large / diverse countries.
Coastal China has per capita GDP comparable to Taiwan. Interior provinces is much poorer, with 600M living on 1000RMB/month or 2000USD/year, on the level of Haiti. In aggregate China is a middle income country with per capita GDP of Iraq.
The Chinese system harnesses this income disparity + 1.4 Billion people to concentrate talent, wealth and development systematically rival developed nations. That's just a factor of scale. Quantity has a quality of its own if properly coordinated, which is basically broad TLDR of Chinese history up to industrialization. I suppose people didn't think China could catch up this fast.