So we now claim Oda restricted Zoro from using haki! What for? Cuz he was afraid Zoro would end the fight in 3 moves?
Same reason why Luffy didnt go G4 from the start.
Same reason why Kaidou jobbed in Dragon form against the RT5.
Same reason why Killer didn't use his Sound based arsenal from the start but used Zanshu Claw when he did not even have Ryou against Kaidou. In that very same sequence as Zoro's Onigiri.
Same reason why no character in OP goes all out from the get go.
But lets single out Zoro alone.
Oda's job is to write an engaging fight, not to make characters fight logically all the time. The moment Zoro started hardening his swords with COA, he was cutting Kaidou effortlessly. Just like how Killer was damaging Kaidou effortlessly once he started using his Sound based arsenal.