They probably used the powder from fireworks, a big no-no. That is flash powder, and it is more destructive than black powder. In making improvised ammunition, flash powder is not to be used in loading cartridges because it will explode the gun.
In Junior High School, early 70’s, the Metal Shop teacher would set off 3 or 4 Acetylene Balloon Bombs to remind everyone what happened if you left the gas on for your torch. We did this Every year in school, had a blast doing it at home!!
We tried something like this the summer after 6th grade.
Mounted a piece of aluminum pipe on a wooden block, with one end of the pipe plugged with a cork or wood or something.
The pipe was maybe an inch and half in diameter and 10 inches long.
We jammed it full of sparklers, figuring it would send out a mass of .....sparkles.
Instead of “sparkles” we got a solid, roaring jet of flame blasting 4 feet straight out of our “cannon”. It sounded like rocket motor. The pipe was glowing cherry red, the wooden base was charred and the sidewalk was black. It was a frigging flamethrower.
Sad. RIP.