Why a torpedo breaking a ship's back still sinks it through flooding

What about a torpedo that breaks a ship's back and splits it in half?

The break creates holes that cannot be closed, allowing the flooding that sinks the ship.

Breaking a ship into sections creates openings that the crew cannot close. Water enters through those openings, so structural failure still fits the explanation that uncontrolled flooding sinks the vessel. The distinction is between the weapon's immediate damage and the resulting loss of buoyancy.