Kuzan was potrayed to have raw (haki off) punching strenght relative to Prime Garp (or at very least the version of Garp from 12 years ago considering the flaskback timeline). This is not hyperbole, this is literally how the flashback presented it with Kuzan's punch slowly progressing to making the exact same sound as those of Garp's.
Based on that point Garp is clearly not some kind of untouchable god that can't be matched by any other marine for Oda.
Garp obviously has more haki prowess going for him, but Kuzan has a busted logia that is fully mastered.
I still have Prime Garp above Kuzan, but Kuzan being equal or surpassing him would not be surprise me either.
If you believe brawling is Kuzan main fighting style, that Garp implying twice that Kuzan might have went soft against his former mentor didn't mean anything and these were just taunts, and that he absolutely went all out in Hachinosu, then yeah it'd be hard for him to be Prime Garp level
But Kuzan having shown his full arsenal seems unlikely to me, even more after that final spread presenting him as being one of the important players of the final saga. Oda isn't really the type to have characters show everything they have before it's their main/final moment. (We had to wait til Onigashima to learn that Big Mom could use souls on herself to power herself up and heal herself despite spending over 80 chapters with her on WCI)