For what it's worth: back in the day (primitive times
) the horizontal punch was described as only a training punch and never anything else, whatever sort of meaning and expostion of information was otherwise attached to it by various instructors. Just a training punch, even when it was explained that the only possible path for it to take as any sort of practical strike was as a decending punch, downward so that the first two knuckles would be the leading portion of the fist. So, no big mystery, just "If you use this, you have to punch down, like this".
The vertical punch was promoted as the punch to use when the path was horizontal to the ground and possibly at a slight angle upwards. Again, no big mystery, just the attempt to hit with the stronger/larger knuckles. Training punches were done in the horse stance or in simple training step through punches, and of course in the respective forms, and vertical punches done in the stance shifting "motion" of motion Kenpo techniques.
Just to add: rightly or wrongly, we "wise" students, back in the mists of time used to judge other styles, other practioners by this as one criterion of their training or understanding. We found it kinda laughable that an opponent would try to use the horizontal fist when sparring, etc.