I don't think the HTV-2 uses (or used rather, since this was in 2011) a scramjet though, but uses a rocket engine. The HTV-3X was supposed to use a scramjet it seems, but that was cancelled.
The advantage of the scramjet over a rocket is that you don't have to carry the oxidiser in a tank with you (like a conventional rocket). You get it from the atmosphere. But being inside an atmosphere making thermal management a big challenge, as the article describes.