Except he didnโt.
This is false, given that every enemy Luffy has fought in each arc has been stronger than every enemy Zoro has fough, by a margin that has increased each arc throughout the entire story. The gap has only gotten wider. Hell, in the middle of the series, we were actually given a number for it.
The captains are and always have been vastly superior to their subordinates. This has been demonstrated multiple times, and was even talked about in a roundabout way on Thriller Bark. Theyโre on an entirely different plane than their subordinates are. This is why weโre about to get a recovered Zoro against King while Luffy is going to face Kaido. Theyโre not remotely in the same league.