The freighter blew up at the wrong place. When the 500 pounds of C4 lit off, the explosion came up from the cargo bay, not from under the superstructure where the rooms are. The superstructure is unharmed in the initial blast.

We see nothing else until we see nothing but a debris field. The helicopter isn't hit by a secondary explosion as if the fuel was set off.

How was the superstructure turned to slag when it was untouched by the initial explosion?

How was the steel hull of the ship destroyed when the entire force of the explosion went up through the deck?

please don't tell me it sank really fast. The Losties on the helicopter are watching it.