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.
As I understand, you can't hit those speeds with just a rocket engine. The HTV-2 is considered a rocket glider - it needs a rocket to propel it to altitudes and speeds where the SCRAMJET can kick in (as the scramjet itself has no moving parts).
You can hit those speeds and beyond with a rocket engine. They don't really have a speed limit, they'll keep accelerating something (provided their force is greater than the drag and any other retarding forces) until you turn them off. Eg, a payload into earth orbit, which would be about Mach 25 if there was some atmosphere. Or significantly faster if you're putting a probe on an escape velocity, for example the Pluto probe New Horizons, which was launched with a solar escape velocity that would be equivalent to about Mach 50 (all sea level).
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.
