That's not the reason at all.
The manga showed us Zoro has CoC/ACoC. Zoro acknowledged during his battle with King he has CoC and he knocked out Beast Pirates fodders with his haki
Knocking out people with haki is the main way to tell if someone has CoC haki.
During his battle against King, Zoro improvrd his CoC haki skills and learned how to freely use CoC/ACoC.
Zoro learned how to add to his overwhelming CoA haki.
Zoro learned how to use ACoC without needing to use overwhelming CoA haki and he can use it without needed to use ACoC haki with enma at the same time.
Zoro improved his ACoC output and can use overwhelming ACoC haki.
The reason Oda hasn't listed Zoro as an official CoC user is because the world doesn't know he has it.
Being a CoC user is a big deal and not every CoC users can coated attacks with CoC. Only a few of the strongest can. The Straw Hats having 2 CoC/ACoC users is an even bigger deal and makes them a bigger thread to the world government.
Oda is waiting for the right moment to reveal Zoro's CoC/ACoC powers to the world. It will shock the world. That time is coming soon and it will be like rooftop 2.0. Unlike Luffy and Sanji, Zoro hasn't used his new powers (KoH) on Egghead Island. When Zoro uses that power is when the world will know he's a CoC user. KoH Zoro vs Nusjuro is coming soon.
