Modbus is the equivalent of VGA connector in displays and projectors. It does have a TCP/IP version as well, but it has no security whatsoever. Or any features besides reading/writing a 16-bit value (or a single bit, charmingly referred to as "coil")

Yes, that is likely. Although Danfoss seems to have their own controllers, I think that you are right, that the attack was not an 'Internet connected fridge', but an HVAC or FMS (Facilities Management System) that was connected to the fridges.