so just because It hit Kizaru, it got to be the strongest move under BG what coping does to ppl
It's not because it hit Kizaru.
But the little things you see that the attack carried.
For example, it was a better version than Kaido took when you compare it in a 1-1 Scenario.
Like many said, in the attack Luffy inflated his muscles, had flames, and those sonic waves demonstrating how fast it was.
Also, considering it was Luffy's last attack before reaching his "limit", he did potentially put a lot of effort in a "last resort blow" for the stamina he had.
All in all he might no be the 2nd strongest, but Itself for sure One of Luffy's higher end moves.