His attacks were doing more damage because of all the fighting Kaido had already been through. It's not like that one move allowed everyone to harm Kaido. Kaido hasn't gotten his ass handed to him by anyone yet, not be Luffy, Zoro or Wamato.
I guess I'll just make a post regarding Zoro's Ashura but this is one thing that is very important and basically confirms the obvious.
-Zoro was putting in all the strength he had left into one attack, just all that he had left and that was him with 30 broken bones and struggling to move. I am not sure how much Zoro is nerfed but even if we go by each broken bone, if Zoro with all bones intact fresh put all his strength into one attack, we can say his full power Ashura attack would be 30 times as strong.
-Zoro never complained about not being able to knock Kaido down. When has Zoro ever wanted to trip his opponents down lol? Anytime Zoro's opponent actually falls, they fall for good, unless its Mr1 lol. He was referring to taking Kaido down with him since his body just gave out by blocking the strongest attack in the series.
Not to mention even with 30 broken bones Zoro was actually blocking many hits from Kaido