The length of the cable is 12,800 km.
The speed of light is 300,000 km/sec.
The velocity factor of fiber is about 68%.
The data rate of the cable is 120e12 bits/sec.
The amount of time that the data stays in the cable is 12,800 km / (300,000 km/sec * 0.68) = 62.7 milliseconds. Multiplying that by the data rate of 120e12 bits per second yields about eight terabits or one terabyte. That is the amount of data "stored" in the cable, at any instant, during transit.
It's not much of an addition to the Google/Facebook data cosmoplex, but it is solid state, liquid cooled, highly distributed and largely immune from fires and small meteor strikes.