Well, if there's water (or ice/frost) inside, you probably don't want to go above 100C. You'd turn it into a boiler, and pressure-test the door seals.

I don't know about these particular fridges that got hacked, but most supermarket fridges I've seen around the world either have no doors, or if they do, their doors have wide gaps and no seals (they'd only make it harder for the shoppers to shop, and given the usual foot traffic, they'd wear out completely after a week).

Exceptions I've seen: some top-opened supermarket freezers (particularly with hinged, heavy doors), and most fridges in small shops. I've seen a few that could turn into pressure bombs this way, but I very much doubt these had any networking/remote control capabilities.