You rock, yabob 
Not true. Traditional gunpowder, usually called black, powder is a mechanical mixture of charcoal, sulphur and potassium nitrate. It does not need to be confined if there is a large enough quantity. A thin trail makes a rapidly burning fuse but if it reaches a pile it’s kaboom city. This is one reason explosions at black powder mills are so common even though the powder isn’t contained. It’s common in that industry for the factory to build a replacement mill long before they need it because the explosion is considered almost inevitable. Fortunately most mills are remotely controlled so it’s just a loss of property not life.
Modern smokeless propellants which are not properly called gunpowder are mostly nitrocellulose or a combination of nitrocellulose and nitroglycerine with a deterrent coating to control burning rate. They behave as you describe. They have to be in a pressure conainer such as a cartridge in a firearm (or a metal pipe in a BATFE demonstration) to explode. It isn’t even considered an explosive by the DOT. Even a metal can doesn’t contain pressure enough to cause an explosion. Cans will burst before pressure is high enough to cause an explosion. Doing the same with a can of black powder will cause an explosion.