Sure looks to me like the plugs melted as designed.
The "mushroom" of fire is a convection effect of the stream of acetylene released by the tank.
Makes for a huge fireball, and dramatic picture, but relatively benign release.
If that showed a real explosion, there would be much less fire, and MUCH more destruction. You can still see the wooden pallets stacked around the tank, for the bonfire. Had the tank exploded, they would be quite gone.