The article doesn't mention issues with putting metal - dishes, silverware, twist ties, whatever - in the microwave. IF you know what you're doing, that's pretty harmless. (If not - sparks, fire, and other excitement often results.)
My (limited) understanding is that the shape is a large factor. Something about sharp edges or points concentrating the electric field induced by the microwaves, leading to a high voltage buildup. So forks are a bad idea, while spoons will likely not result in any sparks.
Yeah, Mr. ElectroBOOM has a pretty informative video [1] trying a handful of different configurations of metal objects in a consumer microwave oven. Turns out it's hard to make "interesting" things happen with metal even if you intend to.
Alton Brown used to have a recipe (I'm Just Here for the Food?) for homemade microwave popcorn where you can use a typical metal staple and it won't spark, something to do with the amount of metal, and also actual size of the microwave wavelengths used being bigger than a staple, iirc.