It's an act of war, and invading neighbouring countries is certainly an act of war. Nobody said all acts of war are identical, stop shadow boxing against invented positions nobody holds.
Regardless, "country A sabotaged infrastructure owned by country B" doesn't automatically mean "countries A and B are at war", it's not that simple. But when acts of war happen repeatedly between countries in an area, it's a sign that the area isn't exactly in a time of peace.
There was a pretty detailed breakdown by a dutch newspaper that even identified the Ukrainian commandos that did it. And they even have an arrest warrant on the name of one of the suspects.
And let's be honest here, Ukraine benefits the most from that sabotage.
But from a realpolitik point of view, Russia must somehow be slapped for these sabotage acts.