Modern medicine had methods of treating tetanus before the vaccine. Many diseases have been treated effectively before a vaccine is developed, but the vaccine is easier, more effective, and far cheaper.
So it did. But modern tetanus vaccine hwas completely re-engineered around 1968-70. Up until then it was a 'killed' vaccine based on culturing b.tetanus in horse serum. which occasionally gave rise to a severe allergic reaction known as 'serum sickness'. When the culture medium was changed to a fully synthetic mix and the killed bacteria were fractionated to increase the bacterial membrane content and reduce the level of cytoplasm allergy problems became very rare and the vaccine was claimed to be more effective.