Didn't see this myself but read about it in the paper a few years back.
A guy was brazing a patch on a gas tank and it blew up and took two fingers off the guys hand.
He had filled it with water up to within an inch or so from the surface that he was working on.
The said that because the gas is much lighter than water, the fumes were all concentrated in that small open space on top of the water and it ignited as soon as he put the torch to it.
As always, common sense and a little thought needs to be applied before attempting any repair.
The volume of the tank and the volume of vapor possibly left in the tank needs to be considered. ....Trapping a pocket of combustible vapor is a bomb waiting to happen. .....A small volume of combustible vapor will just go "woosh!" while a large volume will go "BOOM!"
As you stated earlier, exhaust gas from an engine is inert and will not combust, so it is an excellent way to purge a tank before and during repair. ....The small tank in the original poster's picture does not need purging, if filled mostly with water.
I used a plasma cutter to cut up a very large fuel oil tank (oil was removed & tank swabbed) after running the exhaust from a V8 into the tank for 60 minutes.