not a retcon, Gaban already told us that Zoro can use coc and he even saw KoH and though Zoro actually mastered coc. that means KoH is factual speaking acoc, the problem comes with what Gaban said after this that when you unconsiously use it it becomes useless so basically Zoro was using a non effective version of acoc. basically the same way when Luffy unlocked acoc Kaido told him that the way he uses it is still bad.