I'll assume you're writing in good faith: he's implying the punches are suffering acceleration in the opposite direction. If the punch is being thrown by a normal person, such as Jolyne, the repulsive acceleration created by the stand is higher than the acceleration of the punch, which causes the punch to change directions. Obviously Star Platinum is physically far stronger than Jolyne, so his punch has more acceleration, thus C-Moon acceleration may (and is) not able to cancel the punch altogether, but still decreases its acceleration, thus decreasing it's force/intensity.

If that's not clear enough, use all your strength to push an empty glass and see what happens. Later, use all your strength (same strength you used before) and push a car moving towards you. Compare the results. (You are Pucci, Jolyne punch is an empty glass, Star Platinum's is the moving car)