I think the reason Shiki didn't use haki in the movie was similar to why Madara didn't go all out against the five kage. "What adult would go full force when fighting mere children?" Shiki is senile, prideful, and stubborn. He wouldn't use haki against Luffy because if he did, it would be the same as saying that he felt threatened by a rookie from the weakest sea that hasn't even entered the new world.