I think we have apples and oranges again here. By cable it appears you are referring to wire rope and it has a stretch factor of 1%. Nylon, on the other hand, has a stretch factor of 40% at breaking strain. 5/8" dia nylon has a breaking strain of approx. 9600 lbs while 5/8" dia wire rope, depending on construction, has a breaking strain of 30,000 lbs or greater. When wire rope parts it basically just falls in the water as it's only stretched 1%. I don't know if I'd trust my pig to almost 5 tons of snap from a nylon line.
Regardless, the lines mentioned earlier are 8-10" mooring lines (2-5/8" to 3-1/4" dia.) with breaking strains from 146,000 - 226,000 lbs. That's over 70 tons, long tons or short, flying back at you and if it hits a pig I think we'll have a new definition of rag-out of pork.
The vessel in question was the SS Thomas Nelson, a Mariner-class freighter, long consigned to razor blades. If, by chance, she's moth-balled somewhere the indentation is in the lazarette aft, starboard side.
The most common line parted is the spring line and it happens when coming alongside and checking the vessel. The old adage is, "strain it but don't part it" as a parted line has no stopping power. Even on a 20,000 lb boat, if you part a spring line, you are going to have 10 tons of force coming back at you. When ships were converting from manila to synthetic it was a common source of death or dismemberment. Seaman today are well educated on this and never stand in the lead of a line. BTW, the most common injury seemed to be amputation of the leg at about knee height.