Is that a realistic threat model? What does the technology look like to tap into 800Gbit/s of fiber traffic - under water?
The USA were caught tapping cables in the cold war https://en.wikipedia.org/wiki/Operation_Ivy_Bells
Now this doesn't mean that the Russians are right now tapping these cables; they could be simply mapping their precise location etc for later or using them as a training exercise.
The Russians have submarines for this kind of thing, and I bet they're regretting they sent a surface ship. See a list of some of the submarines used for tapping cables at the bottom of the Yandar article.
This is like killing a person with polonium, which is made in just a handful of facilities around the world. Russia wants to cause worry. They want "respect", which mainly means fear.