You are correct, Luffy showed no concern for two Yonko but twerked in sheer terror of Kizaru’s power.
No you did not understand.
Luffy is not simply scared of Kizaru. He was not scared of facing Kaido and Big Mom, he was not scared of facing Kizaru, Akainu and Kuzan.
This reaction is more similar to the reaction Luffy had when he saw Lucci. Similar to the reaction of Shanks realizing what Kid can do. Similar to the reaction that Whitebeard had when he sensed Oden.
Luffy was not expecting to sense Kizaru's presence in Egghead, so he had this reaction. It means Kizaru is at that level but you have to take the context into consideration.
It's not Luffy seeing Kizaru for 5 minutes and being scared. It's Luffy REALIZING Kizaru is in Egghead and reacting accordingly.
This, in no way, means that Luffy is more scared of Kizaru than Kaido and Big Mom (he already knew they were in the rooftop when he walked across them).
It's him noticing that Kizaru, a strong person, is in Egghead, and reacting accordingly.
If you want to believe that Luffy is "simply" scared of Kizaru, then feel free to do so. I'm just trying to let you understand my point of view and warning you of what may happen next chapter.