I think Putin will put a sudden stop to any sudden gas stoppages. Hes not massing the troops for nothing.I agree. In fact, I think that's the excuse he's waiting for. I'm not sure the threat will be enough, but if Ukraine actually stop the flow of gas, I think the Russians will move in.
The article is short on details (although the authors no doubt have the underlying letter in their possession). I would suggest that the reason the passive voice is used in the article is because there is no threat by Ukraine to shut off the gas, but rather a statement of fact: if Ukraine is invaded by Russia, that could tend to put the pipelines in harm’s way.