I don't know if you are missing the context, but you may be missing the point of what I said.
I was not trying to define the two penalties in totality. I was trying to give two specific examples of when a receiver has been impeded, and then to point out that their wasn't much of a distinction. Again, it's two narrow examples.
Strip away the wording for a moment. If a receiver is held while running the route, the penalty is yardage and a first down. If the receiver is interfered with while the ball is in the air (and the pass might be completed or go incomplete, thus PI does not require the element of "preventing a pass completion"), the penalty is yardage and a first down.
Once you eliminate the NFL implications of PI (i.e., a 50 yard incomplete pass would result in a 50 yard spot penalty), then on the collegiate level, the only meaningful difference in the impact of the two different penalties would be yardage. The point of the comparison was not some huge factual debate on why you call one penalty nd not the other, but to point out that the IMPACT of the penalty is largely the same (i.e., why award an automatic first down for defensive holding, if not to create some sort of rough parity between DH on pass plays and PI on pass plays).
HOWEVER, there is a more significant differential IF IF IF the ref calls "defensive holding", but then because of the bizarre wording of the rule, you do NOT get an automatic first down, merely because the QB was forced to run instead of pass. THAT is the bad outcome here.
I can understand why defensive holding (in the eyes of the rulemakers) is "less bad" on a running play than a passing play. I just think that the play where King got hurt was a passing play before he was forced to scramble, and if the wording of the rule led the refs to view it as a running play, I do not believe the rule is operating properly.