alt Hacker NewsWent and found the LTT video. It's unclear what he did there. He said there was a spark, and then he ran outside with his pack. Spot welding the cells isn't usually that fraught.
Yeah burying a thing in sand is legit. Depending on the size of the thing that's on fire, water might be fine. Standard protocol for electronics that catch fire on a plane is to apply water to cool the device and extinguish materials around it, and then to put it in a special fireproof bag with a bunch of water.
The issue isn't the electricity, it's the chemicals that react with water. An insufficiently large volume of water will make things substantially worse, at least temporarily. Larger volumes will be able to rapidly remove enough heat that it will be a net benefit.