Dude look at this:
They clash for several seconds, no skysplit in any sign, only after Xebec released more haki from his body, both got send flying.
No sign of any haki of Garp and Roger after Xebec used his CoC at maximum.
The next panel literally prove that it is only Xebec CoC who caused the skysplit:
There is not coming any black lightning by Garp and Roger, it truly come out by Xebec alone!! All the black lightning who covers the island+ocean, is XEBEC COC.
You literally see that Garp and Roger don't use any CoC here, they got send flying.
Xebec caused the Skysplit on his own, the fact it happen after he released his CoC+send the two flying(who clearly don't have a CoC at that scene) and his black lightning covering the island+ocean, proves that Xebec caused the skysplit on his own.
Don't lie yourself, admit you been wrong and move on.