You can't do anything to hit anyone from there unless they're dumb enough to run out at you
You walk up to them. You fight them when their invincibility runs out and they have to do something, which therefore involves leaving the edge and giving up their invincibility frames. It's not unbeatable. It's not like they're permanently invincible. If you watch, SK gets a lot of hits in on Plank, though not enough to win.
and it does absolutely nothing productive for you to grab the edge while your opponent is on the stage when you're playing as Metaknight.
It does nothing productive for you to go to the left platform on JJapes when you're playing as Falco and they're on the middle bottom - you can't hit them. It does nothing productive to go to the fin in Corneria if they're on the main part of the stage, you can't hit them. It does nothing productive to go to the opposite end of FD and laserspam if they're crouching as Kirby, you can't hit them. Planking is 'unproductive' in exactly the same way that those are unproductive. In any of those cases, if the opponent is stubborn and refuses to fight in a place where he's disadvantaged, the timer will run out, but that's not the usual case, since at any given point there's at least one person who's behind and thus has to go to a disadvantaged position to try to close it.
Stalling on the ledge to gain near infinite invincibility frames specifically so the timer will run out and you win is exactly that: stalling.
The point isn't to run out the timer, the point is to force an approach. Same as any other positioning advantage.
If the player is doing something productive with the strategy then it's a different story, but if they're there purely to run out the timer or purely to avoid being damaged while not doing anything to further the state of the game then it's stalling. Getting ahead of someone to force them to chase you is a strategy; getting ahead of someone and doing nothing but avoid conflict is stalling.
And Planking does not have the intent of avoiding conflict. In case you notice, when SK92 came close to Plank, Plank didn't drop down off the edge and fly underneath the stage to the next edge to run out the timer - that would indeed be stalling, under current rules. He HIT THE OPPONENT and then went back to the same position he was before (the nearest edge), since it was a good position for him to be in. In that same match which gave Planking its name, the timer never ran out.
Planking is just a positional advantage. It's the only place on the stage where Falco's lasers can't hit; ergo, it's the only way MK can force Falco to get near him rather than the other way around. That's its main use. Not running out the timer. Running out the timer is just what happens when a character that isn't used to needing to approach (Falco, because of his lasers) doesn't realize that he's in a position where HE has to approach, and so sits there laserspamming at nothing, waiting for MK to come to him instead of going to MK.
As I said before, if you want Planking to be against the rules, you have to modify them, because it's certainly not against the rules right now. (Heck, you don't even need detailed arguments to see that, just note that at the tournament (the later rounds of a high-level tournament!) where that match happened, Plank was not disqualified, despite the TOs certainly seeing the match and knowing what was going on, nobody was hiding it.)