Whether Kaido is standing, laying down or flying, the toughness of his body doesnt change.

He certainly wont be killing a guy who cannot move on the ground, he will slay him while in combat or in air.
He can slay him right now as well, just like he could have killed him back in ch923 when Kaido was drunk.
All that Zoro needs is one clean slice, no more, no less.