it's not an explosive any more than a popping balloon is.
Yes it is. Aside from the fact that a balloon doesn't involve dangerous sharp or caustic parts flying in all directions, popping a balloon doesn't involve a chemical reaction. This draino bomb does indeed involve a chemical reaction. In that sense it is little different than any other chemical reaction. True, the draino-aluminum reaction is slower as pressure builds up, but in the end there is a dangerous violent burst (intentionally) caused by a chemical reaction.