The conversation:
This is an interesting turn because it showed the player acting very directly to encourage their belief that yes he was mad at their filth and he was going to now teach them what to do to stop his anger. It also is the first turn where they have any rituals to appease him.
Just as importantly he has now split the people up into two villages with their own populations, which should ease the crowding and reduce the chance of disease. These actions will further reinforce their fear and gratitude. It doesn't matter that he didn't cause the disease, or that it will go away by itself when they obey him. The people believe he caused it and removed it so he gets the benefit! Propaganda can be such a wonderful thing!
Another interesting thing to note is that while the player doesn't know it his new village is located at the site of modern Segou, the climate is wetter during this time period but eventually it will dry out if he nobody does anything to change the weather patterns.