I simply do not think this is true. Most moves either clank or have no exchange if they are done the same time in melee. AFAIK port priority in Melee only affected same frame grabs and same frame edge hogs, and smaller things. Not attacks.
Things port priority affects.
2 attacks hitting the same character in oposite directions with the same knockback (determines which way they go).
Grabs happening on the same frame.
Multiple characters picking up an item on the same frame.
Grabbing the ledge on the same frame.
Characters occuping the same exact spot (port closer to 4 is shoved).
I'm probably missing a few, but port priority pretty much applies whenever things that can't be possible at the same time...are trying to happen at the same time. Brawl added port priority to characters getting hit while in a grabbed or grabbing state (which is dumb), but overall port priority is only a minor concern.