Gaban said haki is only useful we can recognize and control it.
Zoro showed us against King he can control it, just didn't recognize.
In the scene we see Zoro release overwhelming CoA haki and then add ACoC haki.
Gaban told Luffy he needs to work on his ability to control ACoC.
People thought Luffy's haki was much stronger than Zoro.
I agree. It definitely feels like this is Oda way of stopping Zoro from outperforming Luffy like what happened on the rooftop.