Another way of looking at things is to imagine if you were to put several ounces of black powder on the ground and light it - it would go "poof", although the burn time would be measurable, and very little pressure gradient would be observed. Repeat the test with the same amount of black powder contained within a sealed container, and now you have a relatively powerful pipe bomb.

The containment provided by the pipe in the second example results in a significant increase in internal pressure, which in turn is what allows the black powder to produce a relatively large explosion. Essentially as the black powder combusts (deflagration in this case) the combustion gases expand (subsonically) until they either rupture the container, or simply reside due to exhaustion of the reactants.

In the case of IronRaven's fire jar, I suspect that the wall thickness toward the end of the square nub is such that it would rupture first and essentially act as a pressure vent before the entire container would rupture and caused any damage or serious injuries. Having the match heads alternating as he does (most likely for increased storage capacity) also reduces the likelyhood of creating as much pressure rise if anything were to ever ignite the matches.

Jim