> Their new coal plants do not run 24/7, they only run at night

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.

Note that China has started building battery banks at massive scale recently.