They already knew Luffy was a threat to them since chapter 1000, which was the shock chapter when Luffy could hurt Kaido with lvl3 hardening, shocking both yonkos, following with Kaido wondering if Luffy could match him in the following chapters. They just underestimated the others. They didn't know Killer could hurt Kaido by bypassing his scales with his sonic moves before he did it. They didn't know Kid and Law could do the same, which is the reason behind Kaido's statement after he got hurt by them, and why BM took Killer out quickly after he hurt Kaido.
Then she sensed the power behind Zoro's enma sword, which is why she told Kaido to look out for the attack, and after the attack missing, pointing out they underestimated them. It was the point of the whole sequence there. With that said, i'm not saying Zoro's attack wasn't dangerous, it was which was the point of BM calling out Kaido for that. It's just that it was less dangerous than the fanscanlation implied it to be
From the fan translation, it made it seem like the attack was a serious threat to Kaido, with BM's shocking expression while warning Kaido to dodge it. Here however, she just warned him to be cautious of the attack because it wasn't an ordinary blade, without it being necessarily being a threat to Kaido, and the overall scene leading towards them recognizing they underestimated the group as a whole
Overall then, what i mean is that Zoro's attack still had a greater reaction comparatively to the others, but was still perceived in the same general realm(hence BM grouping all of them), with it being particularily powerful, capable to injure Kaido a lot, but not being a great threat to him