You have to get the oxidiser JUST right
When I was at school, one of the most popular things I did that nearly got me expelled was filling draws in the Physics lab with gas from the bunsen burners, then lighting them.
A draw completely filled with gas was useless. If you got it to light, a medium-sized flame would add the smell of burning wood varnish to the already incriminating smell of the gas.
My speicality was being good at judging how much gas to put in, so that it mixed with the air in the best ratio. The best I managed blew the draw 4 feet across the lab, just reaching the next bench.
A car filled with patio gas isn't going to go off. You could sit in there flicking your lighter - it wouldn't light and neither would you (although you'd soon be unconscious from lack of oxygen.
If a detonating device actually blew out the windows, allowing the gas to mix with the outside air, AND it kept detonating (or producing some sort of ignition source) for long enough, then it might set off the gas. You'd get a big pop and a pretty fireball. You might get your hair singed within 20 feet or so. The flame might even blow itself out.
The only real danger I can see is the petrol gets lit and then burns for long enough to heat the gas cylinders to the point of exploding. How many people will be standing around watching by that point?
There wer some other horrifying aspects they forgot to mention in the reports. The "device" included dangerous poisons like dioxins (flammable car upholstery), materials intended to injure people at a distance (friable car safety glass) and deadly shrapnel (well a car is mostly made of metal). Never mind the complex data aquisition modules (we ICE is pretty good nowadays).