While I think they did intend to include barrel control, I don't think they intended it to be used by the player. I imagine they did it they way they did so the barrels would shoot down a ladder as you ran toward it, to make it harder. Having the player use this to their advantage was probably an unintended consequence.
I dunno, before yesterday I never knew the formula, but to figure out that you can control the barrels to some extent is not that hard to decipher. I don't imagine the people play testing and creating the game thought it would never be done, surely people were barrel controlling early after release in the arcades. Before yesterday I already had it in my head that its not worth trying to do it on the first board, and that it rarely works on the second, but from there on its definitely noticeable and worth paying attention to.
I would suggest that since there is still a random element to it, that they knew it would be used by the player. If not, why not make it always happen in the most detrimental way at the higher levels?
Like Javeryh said, its that which gives depth to the game from a gameplay standpoint. If the barrels were totally random all the time, the game wouldnt have as much of a learning to it as it does now. Its like a fighting game, when you can get past the special moves, and really get into the hit boxes, counters, timing, linking etc. Its like a whole nother game. At first you think its just running and jumping, but as you progress, get better and more intimate with the game, you realize its about barrel control, screen positioning (rivet and pie levels) and timing (elevators). Running and jumping fades away to childs play. Playing DK with all those elements, barrel control included, provides another game for the hardcore player, the one that is willing to go beyond playing it 5 times and moving on to the next. It provides depth, complexity, and gameplay at a totally higher level than the game first makes noticeable.
Maybe thats by accident or unintended, but its done so well, hell its still a "popular" game today in the retro sense, it seems to have to be by design.