Why can’t Zoro have his big moment after Luffy hás beaten Kaidou? Zoro “nothing happened” moment was after Luffy beating Moria. Sanji “cake” moment was after Luffy beating Katakuri. Did those 2 undermine Luffy’s achievement? Nope. So why should ZKK undermine Lufffy beating Kaidou?
Thriller Bark was about bringing back Brook's shadow (and reclaiming their own), Luffy was the one to do that not Zoro. Zoro didn't kill Moria because Luffy's attacks weren't good enough, he took on an unexpected unrelated threat, that wasn't narratively significant to the goal of that arc.
WCI goal was to rescue Sanji, they achieved that, but there would have been deaths to innocent civilians if not for the cake.
Luffy also achieved his unreleated goal of surpassing YC1 level to go on the road to fifth emperor status, Sanji's cake was completely unrelated to his goal of that arc, wheras ZKK is directly related to Kaido, Luffy's enemy. Did Sanji kill Kat to make sure he couldn't follow them further?
You are narratively undermining Luffy's victory by saying it isn't enough to save Wano from Kaido.
Furthermore this is a 400 chapter specific enemy of Luffy's who he has said he will personally take down in order to become PK, is Luffy going to kill Mihawk to make sure Zoro stays WSS after his win?
Luffy doesn't even want help taking Kaido down from Yamato, smiling at the prospect of the 1vs1, you think he wants his fucking subordinate killing him because he didn't do enough of a good job taking him down?
Do you really think beating Kaido is bigger news to the wider world than killing him? Zoro isn't going to get a feat even COMPARABLE to Luffy's victory, let alone superseding it, taking Luffy's fatigue is not a feat bigger than taking down a shichibukai, baking a cake is not a bigger feat than taking down two yonko commanders.