The tool, which is able to cut lines at depths of up to 4,000 metres (13,123 feet) – twice the maximum operational range of existing subsea communication infrastructure – has been designed specifically for integration with China’s advanced crewed and uncrewed submersibles like the Fendouzhe, or Striver, and the Haidou series.
I don’t think they need to send people down or even complex sensors, they could use a similar dumb tactic, as above, of dragging something heavy for kilometers, until they can confirm that there was a disruption on the network.