They wouldn't have had a DFDR on a 747SP of that vintage, and the lateral loads generated during the spiral dive would likely have rendered the airspeed data somewhat suspect. Said lateral loads would have had far more to contribute to the damage than exceeding Mach 1.
My understanding of that event was that most of the damage was done by the high g's they pulled when they pulled out of the dive.
Speed indications on any subsonic airplane that goes 'supersonic' would be highly suspect - shocks form around the airspeed probes and as I noted, can really cause havoc with some of the readings. During flight test we have special, dedicated instrumentation to measure things like static and total pressures to very high accuracy - something we wouldn't worry about on an airplane that's never supposed to exceed Mach 0.90