Why does that matter lol, even Oden didn't knock Kaido down in human form but scabbards did that lol, they dropped human Kaido on his ass from across the castle and he was on his back.
Umm no he was not buying time for Luffy and, Zoro came to roof top with one goal and one goal only as he himself said.
TO CUT KAIDO DOWN INTO PIECES
And then Zoro failed to do that since he nerfed himself saving Zoro and he failed to as he himself said
TAKE KAIDO DOWN WITH HIMSELF
Viz did a stupid ass wrong translation lol, Zoro gives two shits about knocking anyone down their feat, he already knocked King down his feet with Oni giri, what did that accomplish??