NATO kinetically trying to prevent Russia from damaging subsea infrastructure WITH a formal UN policing mission is also an act of war, its just more clearly not an act of aggression.
Of course, that would also be true of NATO doing so as part of a broader collective defense operation reported to the Security Council, directed against Russia and explicitly aimed at rolling back the Russian (UNGA-condemned) aggression in Ukraine under Article 51 of the UN Charter.
International law can be selectively applied for different party according to different scenarios (relative to different geopolitical power). NATO triggering art5 (self defense) won't make it valid / feasible to trigger at parallel UN art51. RU using UN art51 to target UKR a soveign territory, is also going to be different than NATO / or NATO country using art51 to do whatever they want on non-soverign / international high seas. All of which is to say while international law doesn't matter much to the motivated, not everyone is powerful enough to normalized/destablize with impunity. NATO might, but not without RU security council (trumps UNGA) approval, of course NATO can supercede from UN Charter framework which IIRC that NATO explicitly states they operate within. But then we have NATO going independant of UN, which goes back barrels of worms.
Of course, that would also be true of NATO doing so as part of a broader collective defense operation reported to the Security Council, directed against Russia and explicitly aimed at rolling back the Russian (UNGA-condemned) aggression in Ukraine under Article 51 of the UN Charter.