I think you're taking this stuff too seriously. You don't see the peak of someone's power by training or a skirmish. (I.E. Mihawk vs. Vista, Luffy vs. Lucci, etc.) You see it in all out clash.
Let's not forget even Chinjao made Garp sweat and struggle.
Chinjao could probably do everything Kuzan did and more, but it doesn't mean anything. Garp is still stronger.