It wouldn't surprise me if the NorKors
did torpedo that Southern ship. However, they evidence is that the Southerners recovered most of the torpedo from inside the sunken ship.
Modern torpedoes detonate beneath the target vessel, creating a combination of shock waves and gas bubbles and stuff that force the ship to flex up and down in the middle which snaps the ship's spine and sinks it. Such a torpedo wouldn't be found
inside the target ship.
I suspect the answer to this
is that this applies to
modern torpedoes, and we're talking about North Korea here.
I think they just want attention.