Indeed, it shouldn't be possible to gib an enemy with a dart. There's a specific mechanical reason: when damage brings a creature's hit points to sufficiently low levels (-10 for non-party creatures), a specific death type is applied based on the type of damage used. Exploded gibs for piercing, bludgeoning, and slashing damage. Elemental types for each element, with the ice statue death for cold damage most notable. And just a normal death for missile, poison, and magic damage.
Distance is irrelevant. It's all about the damage type. So ... was that dart somehow using a different damage type?