As we havee seen in Chapter 1002, Kaido appears to sense Kozuki Oden’s “presence” inside Enma.
I think this line might be Oda dropping hints to how Black Blades are created. So far we know that Haki can be sent through a sword and that very talented swordsman are able to do it almost by second nature (because they view the sword as an extension of themselves).
What we might be learning here is that residual amounts of Haki can live within a sword even after the swordsman is no longer touching it. If this is true, it would suggest that repeatedly pouring Haki into a blade could result in a blade accumulating Haki and changing properties as a result — ie. turning into a black blade.
I think Oden specifically chose Enma for Zoro because the sword actively pulls Haki from the one wielding it. So, just as Enma pulled Haki from Zoro when he first wielded it so too did it pull Haki from Oden. And the Haki Enma pulled from them, is somehow transmitted through its attacks.
If Kaido is saying he senses Oden in Zoro’s attacks, he is likely saying Zoro reminds him of Oden and at a subtext level, Oda is saying, this is how black blades are born.