I'm glad your uber players managed it with only 3 PCs. Course, I seriously have to wonder what other bones you threw at them. Your story sounds extremely suspect since the action economy between foes and PCs nearly doubles in your scenario. An extra second level utility power each really doesn't make up for 2 missing PCs.
Ok i ignore that you accuse me of lying, so i explain how it was possible:
No cheating, but rolled up characters with good stats i admit. But using a chokepoint and not running stupidly into a trap.
I handled a free cure light wounds and regeneration (cleric and fighter utility).
Also the ranger picked the aoe lvl 1 encounter power and was a dwarf with a minor action second wind (a blessing when you can attack with twin strike with two waraxes and +2 damage and have +2 defense when you stand in the middle of some foes)
The cleric is an elven strength cleric with good wisdom and healing strike as encounter power using a fullblade
oh, and the fighter was a warforged battlerager with an executioners axe... yes, this could be called cheating since my minions didn`t do so much damage... forgot to mention it *shame on me*
All had action points remaining.
Did i mention the warforged run into the cave. Got to bloodied by ranged minions and retreated. short rest.
Later Kobolds sent out 3 minions and the slinger and the surviving skirmisher to first look what those adventurers are doing, but just when they tried to sneak up from behind. the warforged decided to run in a second time with back up from the ranger and the cleric catching the kobolds somehow flatfooted. when the warforged battlerager arrived in melee with all those minions, he generated temporary hp nearly as fast as they could bring them down.
The shaman couldn´t really use his blast because of friendly fire, but he and the slinger could get some good shots in. before the second wave arrived, they had killed most minions and the slinger and ran out. The goblins from behind arrived and held them in place for only a short time, because the cleric and the ranger mowed tham down. then the dragonshields and irontooth arrrived with little backup and nearly killed my players (although irontooth was first weakened by the ranger daily) and bloodied him with twinstrike. And Avenging flame from the cleric did most of the rest.
Then the warforged went down, flanked by two dragonshields. The ranger went down and the cleric was nearly out too, so i gave them their utility powers from lvl 2.
But still i say: five players not running inside a cave full of kobolds should not die.
In night below (ADnD 2nd edition) there are more than one cave full of enemies where you could run into and die... and if you play shadow run, a run without preperation will certainly kill you.
The main thing in KotS is: you are not under time pressure before you kill the kobolds. The pressure starts after the kill. So you have enough time sneaking up and scout to know how much Kobolds there are inside the cave and prepare.
Your only defense is: it is an introductory adventure. And up to a certain point you are right.
The mistake of the designers were a) assuming the wizard gets picked and can dea with minions quickly and b) after playing and optimizing too long in their playtest they forgot how hard it is to get used to good tactics.