Because he can feel Zoro's CoC haki.
It's funny to me. When Zoro's uses enma, that uses his own haki, and Kaido states he feels a strange haki, as in unfamiliar haki, some claims Zoro is uses Oden's haki. Even though Kaido never states that and the blacksmith that creates dangerous swords never stated that.
They believe because Kaido is a Yonko he knows more about creating swords and how dangerous swords work than a blacksmith.
Yet, when Kaido, an incredible skills and experience CoC haki user, tells us Zoro has CoC haki now they are questioning Kaido's knowledge in the area. They don't believe he knows what he's talking about.
It never sees a surprise me how far the haters would go to downplay Zoro.