Notice… “he replaced the lid’s bung caps” before welding. This was his (fatal) mistake.
With large enough openings or vents for the gas to escape, no pressure would have built up. Most likely, he would have a jet of flame come out the bung hole. If the bung was too small, the drum might have still ruptured, but perhaps not with deadly force.
Best practice before cutting or welding a closed vessel is to fill it with water. Leave it full, while making the first cut. Once there’s a large enough opening to put your hand through, you can drain the water safely and continue cutting or welding.
AGAIN, if you have a barrel with bungs… why in the world would you put them back in before welding?? Even water turned to steam will burst a closed vessel. Leave the bung off, let the vapors vent!