in my uninformed opinion - its not about the length - its about the environmental conditions on the tether itself and how it will effect a vehicle carrying people-
and also consider that a different sub of similar type and size -a year or so ago -
was pulled by currents to be trapped in the Titanic's propeller for hours
my guess is snags on the wreckage is another factor

Drilling oil in one of fastest ocean currents tests total
Total’s discovery of South Africa’s first oil in deep water could prove to be a bonanza for a country lacking crude reserves of its own and prompt a rush from other majors. That’s if they’re able to solve the engineering challenges of operating in one of the fastest ocean currents in the world...
I worked on this site and drilled the first oil well there. The currents at the surface down to 800 meters are crazy. We would launch the ROV and by the time we got to working depth, we would be 300 meters away from our launch points. Launch and recovery were the worst I've ever experienced and the support boat couldn't even launch. The one time they did launch, they broke their tether and I had to work with the rig to hook up the subsea crane to lift it.
If they find these guys, getting them to the surface would be fast if they have the length of cable. A drillship could do it with a drill pipe with a hook and sling attached to the end, but it's a slower process running pipe.