Not really, the adage is a simplification. You can put metal in a microwave with understanding and consideration (or to recreate a known experiment with a CD, light bulb or CFL) but it’s easier to just teach people it’s a no-no.

In fact, metal mostly reflects microwaves and only heats up substantially if there is a thin enough layer and/or nothing else to absorb the waves. Lone cutlery might get hot but like this, there is nothing the forks will ignite.

Thin metal layers heat up quickly. Aluminum foil and gold-plated china will spark and sublimate nastily.

Metal containers shield the food from microwaves and disrupt standing waves, resulting in uneven heating so they should be avoided.

Closed metal containers (even meshes finer than the wavelength) prevent microwaves from reaching the food at all.

If there is no (reachable) food with water molecules, the cca 1 kW of microwave power has little to absorb it and the box and magnetron heat up significantly. There is a resettable thermal fuse but it may not be fast enough and the magnetron’s big ferrite ring magnet will usually crack first.