In almost every system with failsafes there will be conditions that can bypass them. The goal is not to make it impossible for the unsafe condition to happen, but to make it so that in the expected uses the failure will not happen.
In this case it's a domestic microwave and the mainboard is housed inside the electronics enclosure, so covering the whole mainboard in salt water is not an expected occurrence in a domestic kitchen.
But there are ~1 billion microwaves in the world... I'm sure it has happened somewhere. As a designer of a billion-sold device, your job is to make sure that the expected number of people harmed by your device is substantially less than one, which gets really hard when all the risks are multiplied by 1e9.