Defrost mode? That's all I got.

I would guess they can't reach that temperature, and setting the thermostat that high effectively turns it off.

Why does a freezer need to be on a network, let alone a publicly accessible one?

Danfoss said some systems targeted in the Netherlands remained connected to unsecured networks and were still using default passwords.

Speculating, but I would imagine the attackers were able to either override the settings using default or easily guessed passwords, or perhaps even replacing the firmware or programming to spoil the food. Industrial controllers are not the most secure devices. That world is due for a reality check.