The "Yi Peng 3", a Chinese ship that parted from a Russian Port, has been located near both cables just before they were cut. The ship was detained in NATO water, and now faces an investigation.
Currently, all points to a deliberate act of hybrid warfare
They can measure the location of the break to centimeters by timing how long a light pulse takes to reflect back to the emitter. It is called time-domain reflectometry.
Why are you assuming that would be released publicly? The person you are discussing this with is simply informing you of the existence and availability of the technology you're asking about.
They now the distance to the break from one end. They then use that with a map of the cable to determine the lat and long of the break and send a ship to fix it.