Yes, but "below radar range" in this context means below the horizon, which even from the top of a ship is only 15 km away or so. At mach 4 a Sunburn can cover that distance pretty quickly. I'm sure the low radar visibility make a big difference when hiding from AWACs on the approach, but since radar returns scale with the fourth power of distance I don't see how a missile could possibly have such a low cross section as to be invisible at as close a range as 15 km.