What im trying to say in this video is that Luffy unleashed the advance king’s haki during the doflamingo fight, thats how he knew that this power is the only one that can hurt kaido he didnt just guess this because he already experience this power himself when he fought doflamingo.
So this is what i think, Doflamingo expected to get hit by the king kong gun but he didnt expected that Luffy somehow unleashed without Luffy knowing it that he coat his kingkong gun with the kings haki.. so when doflamingo got hit by the king kong so hard it was the king’s haki and the force of the king kong gun that actually knocked doflamingo inside out.
Only later Luffy realized this… what do you guys think?