I have... I was one of the few who stayed awake during chemistry class. And I have done years of experimentation with Hydroxy on vehicles. Boiling water does indeed produce byproduct gases of Hydrogen and oxygen. Especially at instant extreme temperatures.
But let me ask you this... How does an ELECTRIC arc welder work? Are positive and negative electrodes and a current involved? Could it also create electrolysis aside from the heat boiling it? Of course... Just as you yourself say here.
But since you pointed out it was not submerged welding. If they were not welding on that bottom seam and submerged on the inside of the tank my assumption does not apply. even though totally possible with real science to back it up. If they were not welding on that bottom seam and submerged on the inside of the tank my assumption does not apply.
Hydrogen + Chlorine gas + Oxygen + UV light = A big bang... I just gave you the video... Watch it...
You could arc weld all day in salt water in an enclosed tank and not make enough oxygen and hydrogen to fill a balloon let alone blow a 1.5 million gallon water storage tank 70 feet into the air.
If you do not believe me put some salt water with a couple electrodes with something to hold them apart into a flask. Use a stopper to with a glass tube going to another stopper you can put a balloon on. Seal the wires in the stopper going to your electrodes. Plug your electrodes into an outlet which is 4 times the voltage used when welding and see how long it takes to fill the balloon. I will give you a clue it is going to take a while and your solution will heat up depending on how much solution you are using and how much distance is between your electrodes.
And yes I have done this... you should try it.