I wish the article had more information about the chain itself. I've seen this done with diamond cutters on wire rope (like used for the Kursk) but not with chain. From the look of it, the chain has the outside of the links coated with abrasive?
I assume it's just a big f-ing chain and they hardfaced the crap out of it. This is the salvage industry so they tend to go for low buck solutions like that for a variety of reasons.
From the photos it doesn't look like the chain is abrading the steel of the ship, more tearing it in the way you can tear aluminium foil with your hands.
I wish the article had more information about the chain itself. I've seen this done with diamond cutters on wire rope (like used for the Kursk) but not with chain. From the look of it, the chain has the outside of the links coated with abrasive?