That simply is not true. Punching hard is a function of technique and balance and making use of leverage; while some people are, it seems, born with a natural affinity for these things, they can all be taught. Maybe your trainer doesn't know how to teach them?
"One punch KO power" is the biggest con in boxing. Anybody that spends hours practicing punching has the capability to turn the tide of a fight or end a fight with one punch. It happens more often to some fighters than to others due to style, and abilities that have nothing to do at all with punching hard. Such as timing and the ability to set up an opponent, to get them in a position where landing a hard punch is possible.
The hardest puncher doesn't always win, probably doesn't even usually win, I would think. I would bet on the smartest fighter first, then the best conditioned.
Pay a lot of attention to your punching technique and, especially, your feet. Getting them too far apart will, guaranteed, make you slap and push your punches. Turn your hips and your shoulders and use that force to drive your blows. Throw short punches.