I have suspected the sub’s purpose was for tapping into the cable communication traffic when I first read about the incident. I have no idea how it is done. Perhaps a high tech sensor placed on the cable is able to extract all of the contents of the communication signals. With sensors deep under the ocean, there is little background noise which makes a sensor’s job that much easier.
That "might" be possible with older cables whose operating mode is electrical. Fiber optic cables...virtually impossible. I know of no mechanism by which it might be done. Either sort of cable can be cut, but tapping the signal is another story.
re: “Perhaps a high tech sensor placed on the cable is able to extract all of the contents of the communication signals.”
Probably not. Fiber-optic or solid-shielded coaxial cable puts out FAR less signal than thermally-induced local noise generated at the atomic level; after sufficient amplification, all you would have is “white noise” across the spectrum.