That is correct. IBM type elements are made of plastic, but plated with a very thin layer of metal. From a 1981 article: "the type element was functionally plated using an electroplating process that enabled the metal skin to adhere to the plastic with tremendous tenacity. Even when the plastic contracted at a temperature of -75C, the metal skin did not break off. Selection of the plastic involved extensive tests on no fewer than twenty-five types of plastics for impact strength, resistance to acids, shrinkage, and molding adaptability."

my parents had a selectric which i used as a kid... iirc, the ball had some weight to it, and a metallic finish -- maybe there was a lead weight or something inside of it?