Partly true as the explosion was augmented by the stocks of nearby ordonance, fueled aircraft and so on, so the single bomb did start the crippling, but did not cause it on its own. A single 1000 lbs bomb with a 303 lb (137 kg) explosive warhead (AN-M59) is not enough to both penetrate Soryu's lower hangardeck, which was lightly armored, but still armored. Note the weapon itself did not penetrate the lower hangardeck, so all had to be done by the blast alone. Putting additional explosives to it in the form of stocks of ordonance nearby as well as armed and fueled aircraft, it is entirely possible to cause massive structural damage including breaking bulkheads and decks, armored or not.According to Nihon Kaigun: http://www.combinedfleet.com/soryu.htm
- 1025-1026 While continuing launch preparations against a sighted enemy carrier, attacked by thirteen enemy dive bombers. Two direct bomb hits are sustained, one on the forward elevator, the other aft the second elevator, setting off furious fires and induced explosions among the armed and fueled aircraft.
- 1029 A 1,000 pound bomb hit amidships abreast the stacks, the bomb or its blast effects apparently penetrating to explode on the lower hangar deck starting fires amid the aircraft returned from the Midway strike. The explosion also damaged the upper section of the boiler rooms and shattered steam lines. With the main supply line shattered, steam to drive the turbines escaped. SORYU's engines abruptly ceased and she lost all propulsion on both sides. Plane-guard destroyer ISOKAZE near-missed off fantail.
This seems to indicate that the engines were taken out by a direct hit. You are right though that she took 3 bomb hits.
That was a 1,000-pound weapon, though. Makes me wonder: if Soryu only took that bomb hit with no refuelling or rearming airplanes on board, how would she be salvaged? The damage does not seem to be fatal in of itself, and only involves loss of propulsion. I think power to the weapons should be around. Towing her, perhaps?