That's a strange comment. You can't pull wrong, because the all the mobs in a group will link together when you pull any one of them. It's certainly not outright wrong to not pull something that doesn't link.
Qarn-specific notes:
In Qarn, I suppose there are a handful of pulls that have just 1 or 2 monsters in them, and they might assume those are supposed to be combined into another pull, but it's entirely up to you whether you pull them together or not.
In one room early on, there's a couple of bees and the Qarn Facer. I suspect this was the room your party member complained about. You can group them all up or not, it's not wrong one way or the other. On one hand, if they're grouped up, it's probably faster. These days, I always group that room all together in one pull unless my party had somehow shown themselves to be woefully incompetent already. Power creep over the various expansions have made old dungeons laughably easy. On the other hand, if you don't group them up, you don't have to keep your eye on two sources of Final Sting, and watch for the Qarn Facer to run off if it gets low. Any of those situations are worthy of a stun, and you only have one stun (unless you're a PLD). Again, you do you, it's not wrong one way or the other.
Later on in the dungeon, there's a room with 3 spiders and a ghost. The ghost is in a pull group all by itself, but it's probably easiest to just combine it with the spiders, but again, it's not wrong to do it one way or the other.
Bonus Round: I see that DPS said "its Fruit-Flame," referring to the slabs you place in the thing before the final boss. It's actually Flame on the left, and Fruit on the right, so they were wrong about that too.