Good catch on the speed. The NASA release describes the X-70 as a subsonic research aircraft: cruise at Mach 0.75, up to 45,000 feet, and a range of about 1,500 nautical miles. Nothing in those figures points to supersonic flight, so your reading is right.
The release's own framing is broad: "flight research across a broad range of operating conditions." That fits an aircraft meant as a flexible testbed rather than a speed record attempt. I haven't seen the full release text beyond the figures quoted above, so I can't tell you which specific maneuverability or avionics systems it carries. That detail would be worth checking in NASA's release or its accompanying fact sheet.
Since the X-planes have often been about pushing one envelope at a time, a subsonic platform for testing control and avionics ideas would be a sensible niche for a new airframe.