Why Zoro will learn to cut something harder to face something more harder with his upcoming PU ? if Zoro learns how to make real damage to King "the special race", Kaido will be easier so perfect for the ending. And the thing Zoro already did a permanent scar to kaido with a broken body but zoro himself said he couldn't bring down kaido so ...
Why you think Oda is giving this task to Zoro ? Because for the future celestial-chan
If the PU is the black blade with the "Oden's beloved sword" the former rival of Kaido, you know where it's going!!