It probably wouldn’t run because the air its ingesting to cool is already cold, but if you bypassed the thermostat, sure.

Because they’re not designed to work that way I’m sure their efficiency would be low/non existent in actually cold temps.

Early heat pumps could only work to like 30F or so. New ones are much, much better.

Well it would probably "pee" on your floor. Since typically window unit air conditioners have condensation water tube going to outside edge and just letting it drip out. The condensation water after all has to go somewhere. Minisplits actually usually have a third line going out for that along with the refrigerant lines. Fluid in, fluid return and then condensate waterline for, when the inside is cooling and thus generating condensation.

Otherwise? It probably isn't winterized, but theoretically.... heat pumping is heat pumping. It would try to cool the outside air and thus heat inside. Though its thermostats probably would need tweaking. Don't think those have setting for "please cool down to -15C". Meaning in practice it would never run, since ambient already is constantly below its minimum temperature. Though as I remember most arcane old units didn't have such fancy feature as thermostat. You just turned them on, they would pump at their full capacity constantly and you were yourself supposed to be the thermostat by turning the power switch off, when you didn't want any more cooling.