What I describe is a synopsis of the plot as described in the original adventure. It even includes advice on how to get the party to raid the lower caverns first.
Now it is possible some parties used a different method - such as pitting the Orcs against one another or the Goblins and Hobgoblins against the Orcs, to have a more creative way of achieving the slaughter. The adventure details how this could happen too:
"[the PCs] might be able to set tribes to fighting one another, and then the adventurers can take advantage of the weakened state of the feuding humanoids. Be careful to handle this whole thing properly; it isa device you may use to aid players who are few in number but with a high level of playing skill. It will make it too easy if there are many players,"
Sure, but that is not the plot as presented. Here is a couple sentences from the background to read to the players before they roll up their characters:
"Always the forces of Chaos press upon its borders, seeking to enslave its populace, rape its riches, and steal its treasures ..... Yet, there are always certain exceptional and brave members of humanity, as well as similar individuals among its allies — dwarves, elves, and halflings — who rise above the common level and join battle to stave off the darkness which would other wise overwhelm the land. Bold adventurers from the Realm set off for the Borderlands to seek their fortune. It is these adventurers who, provided they survive the challenge, carry the battle to the enemy. Such adventurers meet the forces of Chaos in a testing ground where only the fittest will return to relate the tale. .... Ahead, up the winding road, atop a sheerwailed mount of stone, looms the great KEEP. Here, at one of civilization’s strongholds between good lands and bad, you will base yourselves and equip for forays against the wicked monsters who lurk in the wilds. Somewhere nearby, amidst the dark forests and tangled fens, are the Caves of Chaos where fell creatures lie in wait"
Also among the DM notes the module details what to do after "the last minion of darkness falls to [the PCs] might" and states that that the courtyard in the caves is littered with skulls.
It is what it is. My claim was based on the plot in the original module, not how some theoretical group might play it differently than intended. I think that claim is well supported and I stand by it.