Zoro has taken one attack from a yonko and might already be seriously injured, had one attack fail to damage kaido or even knock him back, and missed one of his own attacks after needing a mobility assist. and somehow he is yonko level? Zoro's serious fight with Killer and training with Enma have boosted him since the skip.
Luffy is flying around the battlefield, having to dodge and tank attacks, has been the one who has damaged Kaido the most so far.
Zoro isn't yonko level, Law isn't yonko level, Kid and Killer aren't yonko level. Luffy is taking the lions share of the attacks and doing most of the counter attacks. And Luffy arguably isn't quite yonko level.
Zoro doesn't have future sight, he doesn't have internal destruction, he can't fly and he can't tank boro breaths as easily as Luffy. He can do good slashes that slow dragon kaido can dodge.