But the thing is, Zoro learnt zero entirely *new* swordsmanship techniques from pre TS to post TS but learnt Haki. We even see a full fledged Flashback of Mihawk teaching him Haki, specifically CoA.
Okay since you're the detailed type, i'll try to be detailed about my perspective here:
- Zoro learned haki in a specific part of flashback tho, not during the entirety of TS. Obviously you can integrate haki with your swordsmanship to complement each other. It doesn't nullify the difference of Rayleigh's teaching vs Mihawk's teaching.
- Zoro's techniques got stronger, not necessarily due to haki. We just see his technique become bigger, his tornadoes become longer in duration, his flying slashes become longer, is it CoA lengthening the distance or prolonging the tornado Tatsumaki? Of course not, the basics and advanced levels of his sword technique and strength is certainly better compared to pre-TS, not solely CoA.
- Really no one even mentioned that Mihawk the WSS having the strongest haki. At marineford, a guy mentioned Mihawk's "slash" as the world's strongest slash. Is peak CoC applied as a slash? No, the strongest CoC infusion until now is applied as crushing attacks making no-touch zone.
- Mihawk didn't even teach Zoro CoC. Even the burden of proof is needed for Mihawk being able of doing CoC. Mihawk's teaching was all CoA. While Ray taught CoC, something Mihawk would emphasize to Zoro if only he is a master at it. But seeing Zoro's reply to Kaidou ("what do you mean CoC? I just willpower it with all my strength"), he wasnt taught CoC, implying Mihawk is not a CoC master.
- No one mention Roger as WSS at his time, Kaidou hyped Roger's haki instead of him being the WSS, WB hyped Shanks' CoC instead of him being a master swordsman, Vergo saying "haki user" as a separate term
- Zoro gave himself "King of Hell" instead of saying "now i'm one step closer to take WSS throne" or "i' a stronger swordsman now" etc once he mastered CoC, thus giving even more impression that haki is its own thing. Rayleigh saying since the first time that haki is an energy. Using haki is using that energy thus making haki as an independent power.
- Kaidou recognize Shanks as the only living person capable of matching him, while Mihawk didnt get that recognition. This is a clear advantage in Shanks' powerlevel portrayal compared to Mihawk, just like how Mihawk has his WSS title as his portrayal advantage over Shanks.
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Tldr:
- powerlevel-wise, i don't agree about WSS status as the strongest in haki, sincea character's strength as a haki user is its own term.
- narrative-wise, it's always Luffy and his hardest opponents that get the best haki. Always been the case.
- since the beginning, and especially now, Shanks is portrayed as the best CoC user in OP.
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My conclusion: Shanks being a weaker swordsman than WSS doesn't make him weaker than Mihawk in a fight, since until now Shanks is portrayed to have more powerful haki especially CoC.