garp punching him won't make him punch like garp. if it is like that why is helmempo not also a boxer like gar and coby.
Oda took real life inspiration here. Garp can tell him how to punch show him the technique etc. then proceed to pretend he's in actual fight.
Garp's punches aren't strong to Garp, since he can punch much harder. This isn't the standard to raise top fighters, but some of them came to be that way.
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The problem here is your thinking is too limited to the McDojo approach. World champs aren't raised and trained that way.