China tends to build things they don't use or are under utilised due to weird incentives at the province and national level. This include coal power plants.
> In 2020 over 40% of plants were estimated to be running at a net loss and new plants may become stranded assets. In 2021 some plants were reported close to bankruptcy due to being forbidden to raise electricity prices in line with high coal prices.
Beyond trying to artificially inflate their productivity figures, another possible explanation for their weird stockpiling is that these resources could be used in a war time. Nuclear winter? Just turn the coal power stations back on.