How would you even build a propane bomb? For a propane explosion, you need to have a bunch of propane already mixed with a bunch of oxygen (and then something to light it). A leaking tank/hose inside an underventilated room will build up enough propane, mixed with the air in the room, to go boom nicely if you come along at the right time and throw in a match/spark, but that’s hard to translate to a bomb.
The only way I could think of is a propane tank with some kind of timer-spark mechanism that will start making sparks in, say, 2 hours. Put the tank in a closet, start the timer, open the tank valve, close the closet door, and in a couple hours you might get a boom. But if someone opens the door in the meantime or the closet is too big or well ventilated, the gas is going to dissapate and you end up with no or small boom. If the spark goes off too early, you get only a small boom plus maybe a burning jet of gas. If the spark goes off too late, or you’ve got too small a closet, you don’t get a boom because there’s no oxygen left and propane can’t burn or explode by itself. So, I guess possible, but it’s not surprising they couldn’t pull it off.