I mean, Kizaru can be KOed. He isnt Kaido. However, it doesnt mean the Admiral will stay down for good, nor does it mean we cant have a real 1v1 later when he recovers.
This is a good way to take both Luffy and Kizaru out of commission. It was a small chance Oda would let Luffy lose directly right after becoming a Yonko, but he also needed to put a pause on Kizaru trying to assassinate VP.