I don't actually believe the whole Kizaru is TKO'd btw. I'm not that much of a backstabber
KO i can see by virtue of Luffy also being at his G5 limit, but definitely not put permanently.
Said it before, but usually Oda has Luffy either distracted (Doflamingo), Running away (Katakuri), or losing (Kaido) in order to resolve other things.
In this case, being a Yonko, he probably cant have Kizaru "win" and beat the Yonko right after receiving the title. It doesnt play well. Being made of Light, Luffy cant just run or let Kizaru run.
So really the best way to postpone their fight is to knock him out temporarily while Luffy powers down to recover.