I see your point.
Webster's definitions:
Flammable: capable of being easily ignited and of burning quickly
Explosive: relating to, characterized by, or operated by explosion (a large-scale, rapid, or spectacular expansion or bursting out or forth)
In short, it's flammable if it can easily be set on fire. It's explosive if the combustion results in a large/quick expansion of volume.
Section 3 of the MSDS states: "Extremely flammable liquified gas".
Section 5 states: "exposure to fire may cause containers to rupture/explode"
Ok, so when propane burns, it doesn't expand quickly in volume, but it does expand. (the section 5 note probably applies to all tanks anyways). We certainly wouldn't want a propane tank to rupture.
So the fire dept must have opened the tank to let some pressure out (hey look, they did!).
Again, in all fairness, there is a very narrow range of conditions required to light up propane. Even when it is lit, it will not rapidly expand in volume.
Here's an example of how to light up propane (beyond the torch...).