I don't think that it's true at all. The large majority of punching power comes from proper technique.
In the old days some guys just naturally had good form and became heavy hitters.
Klitschko's and Lewis prove that a properly throw punch will do the job.

There are some guys tho' who seem to defy the logic. Big George was one of them who threw slow looking clubbing punches. But man did he have heavy hands!