The simple answer could be the fact that the aura he uses could be like blood and could contain unknown, smaller minerals, like steel molecules. Steel is a mix of carbon which is obtainable from the air, and the iron could just be one of the common components of a lot of things as its in blood, so since Lucarios aura is an energy, it could possibly fuse the carbon with the iron forming steel components in the aura. I'm not really great with pokemon science so idk if the aura actually counts towards Lucarios type.