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It's important to note in this scene that Luffy has never actually sensed Kizaru's power before this.
During Sabaody and Marineford, Luffy had no control over his Haki, and therefore couldn't sense Kizaru's power/voice.
If Luffy knew who it was, he would have just said so.
This is not Luffy being afraid of an Admiral coming.
This is Luffy being afraid because HE CAN FEEL the raw power of Kizaru coming. This is an honest reaction purely to how strong Kizaru is, not to his title/position.
Luffy respects strength, not titles or authority, so his reaction here says a lot.