That does seem to make the most sense.
They had the personal motivation to hurt the Russians plus the strategic motivation to keep the Germans from figuring they can end their upcoming cold winter with a stroke of a pen and have the Russian gas flowing tomorrow. I don’t know how long it will take to fix the pipeline, but it seems likely that the Germans couldn’t expect it to be back in operation before the worst of the coming winter is over. So the benefit to the Germans of defecting now from the anti-Russian coalition is reduced.
Plus it’s a sign to the Germans that other pipelines could be blown up. If you can set off three bombs 200 feet under the sea, you can probably shoot an anti-tank missile at an above ground pipeline.
For the Ukrainians, it wasn’t much of an escalation — they’ve already been blowing up stuff in Russia, and this didn’t kill anybody.
As for the means, it’s not that hard to obtain a smallish commercial vessel at a port somewhere in the north Atlantic. You might be able to just lower mines down to the pipeline, but you’d probably want divers to place them precisely. Ukraine owned offshore oil wells in the Sea of Azov until Putin took Crimea in 2014, so they probably have some experienced divers.
Ukraine could probably pull off this operation for a budget of, say, one million dollars.