Tokugawa couldn't eat the candy/forgive the villagers because he was allied with Oda. Forgiving them would be the same as saying "I'm actually sided with Takeda, so it's all good". With Oda adding the lozenge to the candy it was showing that Oda would not see the forgiveness as betrayal, and that Tokugawa would be ok to let them off the hook.
Posted that in the wrong chapter.