I believe Luffy and Kizaru hit each other with conqueror's coating and that split the skies.
This explains the "9/10" for both Luffy and Kizaru fans and the "fierce fight" in editor's note.
Now what I wonder is, is this the result from the clash from previous chapter 1091, which would imply that
@Kurozumi Wiwi was right in saying it was conqueror's coating, or is it another clash?
I'm really curious now.