???
Shanks is a swordman ,stated in this chapter.
And the Marine later says Mihawk the world strongest swordman what do you want?
I want nothing, you read the summary however clearly you read only the parts that suit your agenda
I think that was explained to you.
All that was stated I'n the manga is that after countless battles a swordsman has the ability to blacken his blade
No where was it stated that it required strong and powerful Haki to do.
Even if we believe in the common headcannon that COA is the main reason for blacken blades it still doesn't affect difference between of Mihawk and Shanks
According to Oda, Zoro specializes in CoA while Luffy specializes CoC. Despite having better CoA, Zoro's overall haki is still weaker than Luffy's even without adding Adv CoO to the equation.