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.
If this was problem related to flammable gasses igniting... it was because the welding going on was in an area that was just above the water level and some heavier than air gas like propane was mixed with the right amount of air to combust when a spark or arc above the ignition temperature of the gas ignite the mixture I was the head of a Hazmat Team for many years. We have not been given enough info to know exactly why this happened but your