With military experience he’d know a little about fuel air explosives. Powerful fireworks are sufficient for making such a bomb, though they’re technically difficult (you need to trigger the second trigger explosion after the first has spread out the fuel enough to detonate)

I think he was expecting (in Marvin the Martian’s words) an earth shattering kaboom but didn’t do it right

Done right the amount of fuel would have done serious damage - all the energy released in the fire would have been released instantaneously, with a shock wave