It appears to be resistive when the electric part is being used, but looks like it also takes waste heat from a coal combined heat and power unit, and some other industrial heat sources.

I think the electric heat part is to help provide load balancing so theyre probably aiming for cheap and simple as it'll be used intermittently.

It is possible to have heat pumps at the other end of district heat systems, to let individual units pull heat from the shared source and also feed it back im when in AC mode.