Reading the chapter through the viz properly, you realize that Oda wanted to give due hype is to the enma, who keeps Oden's presence in her. Kaido was startled because he saw the presence of Oden, and Big Mom also realized that it was not an ordinary sword.
Big Mom says '' We underestimate them '', Zoro had no prominence here, he was obviously going to hurt Kaido just like all the other SN's hurt Kaido in this chapter. But Zoro had no superior portrait.