On the one hand, the US explicitly, publicly, and repeatedly threatened to unilaterally stop the pipeline if Russia invaded Ukraine. The US has a clear benefit in the sense that they remove the option for European allies to buy Russian oil. The US also has the ability to do it in the form of ships in the area.

On the other hand, perhaps it's in Putin's interest to destroy his own pipelines for some reason.

The key point is that "they" is the Ukrainian war propaganda machine, which is obviously not a reliable source of news or analysis.

It is also a classic propaganda technique to differenciate between the awful dictator and its poor people/country.

I do not know what happened and if it was deliberate it is not obvious who did it. People should be prudent and not buy into propaganda from either side.

I wonder what would be the bare minimum required to stage an operation like this, if one were willing to take big risks. Like, combat divers operating from an inconspicuous sailing yacht or the like? Sail into position, go down (80 meters should be doable with Trimix), place explosives with timed fuses (set to a few weeks?), go back up, throw the equipment overboard later, sail into Kiel with no one the wiser? Not that such a scenario is very likely, but there have been a lot of crazy stunts in this and other wars.

They have combat divers (one of the special forces units is even the 1st Underwater demolitions Unit, apparently) and the average depth of the Baltic is 50m. So my guess would be: yes they have the capability in principle.

Ukraine is a proxy in a proxy war. Ukraine would have no capability to do this, but USA would.