If diamond is harder than steel, shouldn’t the rock and steel weakness/resistance to each other be reversed?

I see no compelling reason for it, unless we believe a) that Rock Pokémon are made of diamonds or something equally hard, b) that Steel Pokémon are made of ordinary carbon steel and not some kind of bizarre bio-alloy, and c) that Mohs’ hardness is a good indicator of how a Pokémon’s body holds up under the diverse stresses of battle.  On the other hand, I see no compelling reason for the way that relationship works currently either, and Steel-types have too many damn resistances anyway, so sure, go nuts.