So Boccaccio was an incorrigible optimist because he shows how the worst person can make a story that inspires other contemporaries.
On the one hand, Boccaccio shows us how a bad cheater makes other people stronger in their belief–and on the other hand, he lifts the veil and lets us see how we indulge in fictions. But we need these fictions to outgrow ourselves in life. So for Boccaccio there are only stories that help us live better and stories that help us live worse. That is its form of radicalism.