As someone who has first hand witnessed a fair share of microwaves getting wrecked by people microwaving metal in them accidently, that answer is bullshit.

If the metal object gets near the (typically right) side of the microwave, it 100% will arc and at best short out the microwave, but at worst I have first hand seen it cause a fire.

The metal object effectively becomes quite a fair bit charged with electrons from the surrounding air from the microwaves running along its surface. This in turn slowly builds up a negative charge difference between the metal object and the surrounding walls.

At enough of a differential plasma will form and an arc of electricity will go from the metal object to anything it can get close to.

And if that "thing close to it" is the wall of the microwave that houses the actual unit in it, it can short out the electronics sitting in there.

Also, it can seriously harm someone if the microwave isn't grounded properly (and boy is that unfortunately more common than you may think), and they are touching the frame of the microwave, as now they are the ground.