Why is Mulchaly confused about the term break the seal?

Mulcahy tells Robicheaux and Bootsie that a man appeared on Mulcahy's doorstep the previous evening, cautioning the priest not to "break the seal". Mulcahy is certain that he was referring to the "seal" of confession, meaning that whatever a priest hears in the confessional is confidential information. However, he says what Lila Terrebonne revealed to him was said outside the confessional.