I mean, it will definitely cost someone money to go fix the cables. I know that in the US, our insurance explicitly does not cover acts of war (1) so I would be curious as to how insurance covers this.

I suspect that the costs of moving extra bits through other cables is not large. I would assume that Hetzner (and the other companies that own parts of these cables) have peering agreements with other companies and that most of them will not try and take advantage of the cut cables to renegotiate their peering agreements (2). So whatever rates they paid before will still be paid.

1: Because a war creates a problem for the risk pool, it is one of the things that actually can destroy huge amounts of property simultaneously, so it is a risk explicitly separated out and basically impossible to insure against, at least in the US commercial market.

2: Too risky to start renegotiating when your cable can be cut just as easily the next time.