I disagree with the part you said he can't control it.
Based on the manga, he could control it, he just didn't know he's controlling. We can see that in the following things.
Here we see Zoro first release overwhelming CoA haki from his body. The he adds ACoC haki to perform his KoH attack.
He clearly is controlling it separately then CoA haki.
Here Zoro adds ACoC haki when he performs a counter attack the King's attack.
Those scenes both clearly shows a level of control.
If Zoro had no control over it we should see overwhelming CoA and ACoC haki both release at the same time or we should see his CoC/ACoC leak at random times. If Zoro was already using overwhelming haki, what did he think he was releasing? Does Oda honestly expect us to believe Zoro thought he was adding CoA 2.0. haki to his swords that was already coated with CoA haki?
This is just poor writing to stop Zoro from overshadowing Luffy.