That’s baseload! Baseload is load you can’t turn off: the minimum load that’s required in a 24 hour period. It can be fulfilled with non-dispatchable sources, but it need not be. In this case, China is building coal plants to address the baseload that doesn’t go away at night when the solar isn’t producing.
Do you want to hazard a guess as to why?
The answer is because we still do not know how to supply expanding electricity demand with wind, solar and batteries. The side of the story that people on this website love to bring up is the fact that China happens to be the largest builder of solar and wind, which is true, but the increased electricity demand is being met via coal, for the most part.