Kaido's own fans don't understand his character and would rather powerscale than look at the narrative
Sad stuff
No the battle wasn't "already won" when CP0 stepped in
That's the whole point of Kaido being so shocked/disappointed.
He feels like a good fight has been stolen from him
The point of the scene is that the outcome was up in the air and undecided yet, Luffy was probably about to use a KKG version of Over Kong Gun