Luffy and Zoro both shared the Otama experience and went to save her, saw the poverty and everything.
Both got split up,
Luffy bonded further with Otama and people of Okobore town, saw their suffering. It ended with Kaido coming there and destroying it- and though Luffy didn't seem pissed off about it momentarily since his nakama were in trouble, he prob held a grudge over it as he formed bonds.
Zoro bonded with Yasuie and people of Ebisu town as they looked after him and tended to him- it all ended with Yasuie dying in front of Zoro's eyes, which angered him immensely as it was someone he cared for, enough to swear revenge (Luffy didn't even know Yasuie, btw).
In Udon,
Luffy met Hyogoro, learned more about Wano's suffering. Got trust of prisoners and swore to save them.
In Ringo,
Zoro meets Hiyori, whom he spends a good chunk of chapters with, knowing more about her and Wano's suffering- he formed a bond with her as well, much like Luffy to Shirahoshi, or Luffy to Rebecca, and inherited her father's blade (Oden's will, as swords carry souls of dead).
Zoro, in fact, has
more story-driven build-up to fight Kaido than Luffy does as far as development within Wano is concerned. Luffy has a "personal" reason in wanting to become Pirate King, but don't claim Zoro's aren't personal when Kaido caused pain to people he formed a bond with- and indirectly led to their deaths even.
So, yeah, that claim is completely out of ass. Both Zoro and Luffy have equally compelling reasons to personally go after Kaido- and both do.
If you actually want to look into it, and not baselessly shittalk, you can just go through the thread by
@Dr_Professor83 which pretty much coherently lists them all- all the build-up, narrative elements and story elements:
https://worstgen.alwaysdata.net/forum/threads/zoros-unfinished-business-with-kaido.15295/