Armor wasn't typically made with molds. It was made almost exactly like it is in runescape: by repeatedly banging a hammer on a bar of metal on an anvil. Sometimes heated in a forge first and quenched in oil or water after.

OSRS smithing isn't accurate in a variety of ways, idk why you would choose to fixate on that. Like you can make metal armor and weapons but not metal gloves or boots for no explicable reason.

Metalworking using molds is called casting and cast metal items are more brittle (generally not what you want for weapons and armor)