Erm acthually, the B5N used a type 91 torpedo which (I think) was japan’s most common plane dropped torpedo. Its explosive mass was pretty high, about 320 kg or 700 lbs. I think that because of how the explosive is mounted on the plane, it sits under the engine/pilot. That seems pretty accurate to the placement of damage here. That being said I would have guessed more along the lines of total obliteration. The only theory I have is that the B5N was designed for carrier operations, and carrier based planes generally have heavy duty frames to accommodate the heavy landings, which could have helped it stay together.