Went 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.