

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.
Lots of people drink bottled water, soda, beer, or other drinks not immediately connected to the water supply. Furthermore, poisons are unlikely to remain undetected long enough to kill the entire population. While a strong dose of a deadly poison like cyanide can kill in minutes it’s likely to be detected quickly due to how rapidly its effects begin to show up.
A slower-acting, accumulating poison like dimethylmercury could potentially kill more people because its effects don’t show up immediately. On the other hand, the delayed effects of the poison would provide the victims a chance to retaliate against the poisoners.
Either way, it’s a very crude and unfocused attack against a population which is unlikely to achieve any political aim besides wanton destruction and outrage.