That’s not really how it works. Some random Chinese peasant (that’s the vast majority of China’s population) doesn’t produce much CO2. You can add or remove millions of them without significantly impacting coal consumption or CO2 production.

Industry pollutes. Some types pollute more than others.

China has been increasing energy usage across the board at a much higher rate than the population has been growing. It’s a nonsense plan because there’s no reason to think that reducing the population would affect that trend.

While there’s a clear trend of China using more coal there’s just as clear a trend of coal making up a smaller and smaller share of China’s power usage over time. Just about every analysis says they’re solidly on track to completely phase out coal by 2025 and nobody predicts they’ll need to shrink their population to do it.