I read that the gas was already turned off, the only gas coming out was what was still in the pipeline.
While I’ve read the “turned off” claim that doesn’t quite compute. There reportedly was “no flow” because the Krauts hadn’t approved their outlet end of the system. But there would be a large difference between “turned off” and “no flow.” There would still need to be at least some pressure constantly applied to maintain the test pressure. A line(s) more than 400 miles long would have condensation, bleed, etc. such that completely shut would gradually lose pressure. As best I can tell most of the length of the pipes are around 200 to 300 feet deep. 300 feet of water is about 9 atmospheres (about 150 psi ) of external pressure. Left alone the internal pressure would degrade and the external pressure would bleed water in.
We don’t really know what stage of testing the Krauts were in but that implies full test pressure was being applied. Whatever blew could not have exhausted all the gas in the line. All the line above the break elevation would still have a bubble of gas and that could be several hundred miles long. I don’t know this exact system but long pipelines always have emergency stop valves installed, probably every 50 miles or so, such that pressure spikes/drops automatically shut them off. That’s how we keep from losing all the oil in the Alaska pipeline every time a reindeer sticks a horn in it.