It seemed obvious at the time that the Americans were the most likely perpetrators. Everything that has happened since seems to confirm those suspicions, most notably the conspicuous lack of attention and investigation of what is probably the most serious act of war against the EU since its founding.
Hersh's article outlined a plausible but far from watertight case. This was followed by the leaking of 'intelligence' that it was a non-state Ukrainian aligned group that hired boats that would be incapable of carrying enough explosives for the job. As version points out, that has dissolved like so much methane in the Baltic sea.
So, Occam's razor. The US had the means, motive and opportunity. They said they were going to do it, they celebrated publicly afterwards and now nobody wants to investigate it.
Lesson for Europeans? The US can and will attack your critical infrastructure without fear of consequence, and there's nothing you can do about it.