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I still can't believe this many people haven't figured out that the steel typing doesn't strictly mean being made of metal, but that the pokemon has properties in it's body that behave like metal/steel.
Perfect example was stated above, how alolan sandshrew's iced is so compact that it behaves like steel, thus giving the typing.
Even if it isn't actually metal, i the properties of the substance behave like steel, than it gets the same benefits/weaknesses of it.
Mawile's teeth like horns in it's false jaw, are probably as hard as steel. Same with lucario's spikes, most likely protruding bone.
Yes, most pokemon that have a steel typing, it is clearly reflected by being made of some sort of metal on their bodies, but not all of them.