I wouldn't be too worried.
I helped a friend build up a late 90's Cannondale (also aluminum) road frame which has a dent slightly larger than that on the down tube and two very large hard whacks on the top tube, plus several other smaller dents and scratches. It lived a long, hard life before and is living a hard life now. This kid rides the thing like he's trying to break it, he's put on at least 2 or 3 thousand miles, tacoed both wheels and the frame is still holding together.