Luffy as a whole is even less of a threat to Kizaru, so this is a confusing point to raise.
Gorosei aren't great fighters but they are powerful. The reason Gorosei were unable to land attacks on him is because Luffy was literally using his 2 minute hyper overdrive form which places him above top tiers temporarily. If Oda is gonna give Luffy such a shitty drawback and have him still somehow be getting pieced up by people, what even is the point of G5 at that point? Throw that form in the trashcan.
Imo you should be looking at G5 not doing anything to Kizaru with scrutiny, that was the real issue. Kizaru wasn't the one going tryhard using some stamina draining time limited form only to get no real results, he looked quite casual in comparison. The tradeoff has to be Luffy is stronger for this time frame cause he sure as shit wasn't outside of it. But what came as a shock is Luffy himself was just surviving throughout the arc, he wasn't actually able to impose himself on Kizaru the way he should have given all the drawbacks.
It almost gave me flashbacks of G4 against Cracker. Where G5 didn't even look like all that much, which is not a good thing.