One important question that I'm unclear on is how long it takes to fix one of these cables. If it takes months then that is quite a wide window in which an attacker could incrementally take down cables.
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(And if they decide they don't care about plausible deniability, they could use sub-deployed timed mines to take out every cable at once.)
In an actual war, you hit the repair equipment and personnel [1].
(As to the Geneva Conventions note, we're discussing a hypothetical war with Russia. The status quo, including rules of war, are going to be rewritten by the victors.)
The frustrating part of this kind of petty tactic is that bullies can do just enough to annoy and inconvenience their targets, while never quite doing enough to make it worth expending the political capital to hold them to account. From the bully's perspective there's no downside. And if legitimate accidents or rogue actions get portrayed as deliberate then all the better - that just reinforces the bully's reputation as an actor to be feared while further eroding trust in the international institutions that may one day challenge it.