By attack undersea cables the ship are no longer exercising the right of innocent passage, and thus is not protected under the law of the sea.
The whole point of doing it with a tanker was deniability. Doing it with a Russian-flagged ship makes it overt. The anti-escalation logic applies in both direction: Russia wants to sabotage as much as possible without triggering a huge escalation, because they're not sure they can win that either (and nobody could win a nuclear exchange!)
The ship that was boarded, is registered in the Cook Islands (an associated state of New Zealand), owned and operated by a company in United Arab Emirates. And the ship's crew were Georgian and Indian.
Background: https://en.m.wikipedia.org/wiki/Political_status_of_the_Cook...
It looks like the Cook Islands are heavily dependent on financial support from NZ government, and NZ tourists.
That's not to say that nothing can be done, but that it's important to do the right thing, or we risk eroding the few hard-won rules we still have.
China retains the right to board any ship in what it considers its sovereign territory, UNCLOS be damned. A similar reading by Finland would let it legally board any Russian ship transitting "its" straits.
Funny, how all the lectures about the "rules based order" and the "rule of law" from Western necons are shown to be total lies by their own words.
https://www.un.org/depts/los/convention_agreements/texts/unc...
NATO members can and should find some other pretext to stop, board, and search some of those Russian vessels. But if Russia doesn't back down and sends them with Navy escorts what then? It's worth thinking through the various escalation scenarios before acting.