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.
But you omitted the full combination of gasses that might have been created. Also Chlorine gas and possibly even Fluoride gas if the water had fluoride in it. Fluoride also reacts violently with Hydrogen.
I truly think it is possible to create the conditions for an explosion in a container such as this considering the combination. Doesn’t take much Hydrogen and Chlorine for the UV to set it off. You saw it pop with a very small amount in that video.
But while possible, I also think it would be very very rare indeed that these perfect conditions could come about. I learned never to say never years ago when I have seen even stranger (physically impossible) things actually happen beyond all reason and logic.