Look at the most recent chapter and what does it say ?
This explains how the sword works, nothing has changed that.
The source Memphis telling us how to sort works
A blacksmith explained to us how to sword works.
In the latest chapter, Kaido states he feels a strange haki coming from the sword. He is familiar with Oden's haki so it's not Oden haki he feels. Since Zoro's haki is strange and new to him, since he never fough Zoro before, that explains to us why he calls it strange.
After the attack he realized that Zoro has the sword that used to belong to Oden.
It is funny how people believe Kaido, who doesn't even fully understand ryuo, he didn't know how the retainers cut him, somehow knows more about the sword then a blacksmith that told us the history the sword, explains how it works, and has created his own dangerous sword.