Adv. CoC doesn't make Attacks Stronger
It makes them more Effective
Adv. CoC is basically similar to Breath of All Things
When Zoro learned it, it doesn't mean his Attacks are More Powerful
But They deal more Damage against Specific Targets (Such as Steel & Fire)
It's similar to Games like League of Legends
If you have an Attack that deals 100 Damage, and i tell you it's Magic Damage
Then your Opponent will take Less Damage if They have Magic Resistance
But if it's True Damage, then Opponent will take 100 Damage Hit
In this Case, Adv. CoC Counters Haki itself
Meaning when you hit Someone with it, the Damage is Same but it's not only their Bodies that are gonna absorb the blow but also their Haki
Hence why Kizaru wasn't Knocked Out, but He was unable to Continue Fighting, it's like his Haki was Drained (Similar to Luffy Post-Gear 4)
So CoC is for when you have Ability to Land Attacks, it would help you guarantee the Damage & Defeat your Opponent, but if you can't Land Attacks, such as Opponent being Faster than you or have Excellent Guard or other Defensive Ability, then CoC is Useless cuz it won't make your Attacks Stronger, you would still be Outclassed
Non-Gear 5 Luffy cannot Land any Attacks on Kizaru, so regardless if He uses CoC or Not, He still Loses & Result won't change
And the same goes for any Opponent who can Out-Skill Luffy (However, if Opponent is Adv. CoC User, then it must used to Defend)
Whether it's Lucci or Kizaru or Shanks, Luffy cannot Win without relying on Gear 5 (That gives him Access to Infinite Strategies)
However, since Gear 5 is Temporary then He must make sure his Opponent doesn't Outlast him
But again, CoC would be used only to guarantee that his Opponent is Down before Time runs out (To either Win or Force a Draw)
However, against Shanks Level Opponent, it doesn't matter if Luffy uses CoC or Not, He ain't gonna Land a Hit