Medieval technology is vastly more complex, broader in scope, etc. compared to the Stone Age stuff on Primitive Technology. It’s actually extremely challenging to go from scratch like he does and then achieve medieval-level ironworking. He can barely make a few little iron pellets which are excessively-hard (too much carbon) and need further processing to become workable. He is a very long way from building a proper medieval smelter capable of producing pig iron or other cast iron products.