It certainly resembles a rooted clone and not a seedling. The stem seems abnormally large for a plant that's only 15 days from seed. This sort of growth is usually caused by heterosis, a term used to describe the unusually vigorous growth which hybrid plants sometimes experience. It's nothing to be worried about and is actually a trait that breeders strive to achieve. Also, having nodes which are tightly spaced in that fashion will produce clusters of buds once flowered. I'm usually inclined to top plants like that to produce more colas.
