The story is set in 1184, when Salah-al-Din is trying to conquer Jerusalem, in the hands of Christians. Among his councillors is a mullah, a kind of Muslim priest. All mullahs belong to the Shiite part of Islam, while Salah-al-Din was a Sunni. And, since the two confessions hate each other, would have really a Sunni king followed any advice from a Shiite one?
Not really "hate"...but I guess yeah, their views don't really agree, so they don't like each other very much. I guess that there probably are some Sunnis that absolutely despise Shi'ahs (Shi'ites) though, and vice versa. But, I mean, why? They're both Muslims! But also, as you were saying, a Sunni would likely not have a Shiite mullah or imam.