You can also just have a spoon in the microwave too, right? I think It's only from things like crinkles in aluminium foil and the prongs of a fork that produce sparks and arcing inside.
You should also put something (eg. a cup of water) in the oven to absorb some of the microwave energy. The metal doesn't absorb much energy so without an additional absorber the microwaves can get reflected back into the magnetron and overheat the oven.
My microwave oven comes with a metal stand that you're supposed to put into it when doing certain kinds of baking. Likely it's engineered to be massive enough, it has rounded edges everywhere, and is supposedly sized so that there are no pieces that would resonate at 2.4 GHz and thus consume a lot of energy and overheat.
Mehdi Sadaghdar has a great video on this:
https://www.youtube.com/watch?v=OyTmJX_TC84
Kudos to anyone who does actual experiments to test “common knowledge.”