That's true. Maybe because Zoro hadn't tapped into it like Luffy had? Even if he did use it at Baratie, it would have likely been similar to how Kaku could sense a demonic aura coming from him. It may not have been a full blown release of Haoshoku Haki.
I'm not sure though. Mihawk didn't seem to be as dedicated of a mentor as Rayleigh was so I can see him omitting things that he felt were outside of Zoro's skill level.