Honestly it doesn't even have to be a retcon.
Versus Kaido it was a desperate do or die situation. If Luffy lost, everybody dies, the country of Wano is subjected to Kaido's torturous rule and most importantly, Luffy despised Kaido and wanted to beat the shit out of him.
From a story perspective, Kaido was the final antagonist, and Luffy usually passes out only after defeating the final antagonist.
From a story perspective, Kizaru wasn't the final antagonist. Is it do or die? Kinda, but the stakes simply weren't as high on Egghead at that point. Furthermore, Luffy probably felt he did enough to knock Kizaru out of the game so didn't bother forcing himself to continue just to pummel a Kizaru who could no longer move.