No, the point is that they do. The same amount of gas is released whether the barrel is indoors or outdoors. The only difference is how that energy is directed.
The point is that you can't on the one hand say that "oh, it's just destroying the barrel and that's not that strong" and then show that the same same event in a structure can completely level that structure. Yes, containing an explosive in a room causes the energy to be directed against the walls of the structure, but that's still the same energy being released inside the room to the occupants. That pressure doesn't just blow past them. And much of that gas had to have been once contained in large part by the barrel itself.
But let's back up and look at the "barrels aren't that strong so they wouldn't explode with that much force" argument.
Every year people routinely severely injure or maim themselves with firecrackers and salutes. Loss of digits or whole hands is common. Those are a very small amount of powder. M-80s are typically said to hold about 3-5 grams of powder -- flash powder, yes, but still a low explosive powder -- wrapped up in cardboard and sealed with wax or glue. You can't tell me that an iron-bound wooden barrel or keg is less capable of containing pressure than a cardboard tube. Bear in mind, too, that barrels are often designed to contain wine, beer, and similar fermenting liquids. Barrels are capable of holding significant pressure just by being barrels. Powder kegs would likely be constructed to be as water tight as feasible, since wet powder doesn't burn at all.
Similarly, cardboard isn't particularly known for it's ability to be deadly shrapnel, while a wooden barrel would shiver itself. Further, if you're going to make a keg bomb you can do something as simple as pack it with stones, line the outside with nails, or mix in metal scraps.