I also very much doubt it was the Ukrainians in by themselves - as blowing up heavily reinforced pipes 80m under the water is a rather extreme task, but at least they would have had a reasonable motive. Russia was fueling the German economy, and Ukraine would have had a viable concern about Germany prioritizing their own economy over Ukraine. OTOH it seems somewhat obvious at this point that Russia would not have threatened to turn off the gas, so it was a terrible miscalculation by whoever did it.