So much roof space, so little PV. I guess in China they don't do rooftop PV as much because the regulations allow for cheaper installations somewhere on a meadow?

China has more distributed/rooftop solar than the US, percentage-wise. It's hardly like every big building in the US has rooftop solar, and Zhengzhou is more to the north of the country where there is less incident light.

That said a battery factory is a good place to put solar. The final stage of battery manufacturing is several priming charge/discharge cycles which build up resilient layers inside the battery. You can push power into/out of the grid (or use discharging batteries to charge other cells) but having a big DC source nearby is still going to be convenient.