Metal was your assumption. But also metal objects can be safe in a microwave. A metal spoon it is an optimal reflector of microwaves but in this case it is surrounded by water which is an optimal absorber. It's dangerous to heat metal by itself but that is a moot point because it is also dangerous to microwave nothing at all..

I'm not spreading dangerous information, you are assuming dangerous operation. Cooking can be dangerous. But you are doing something akin to telling someone not to cook with oil because it can splatter and cause terrible burns, rather than informing them on safety precautions when cooking with oil. Rather than fear mongering about superheated fluids being an inevitability, you could tell someone to avoid superheating fluids, the simplest way being not overcooking. Which is very easy to do with a microwave because they are predictable and shut off on their own.