Well my (basic) understanding of aerodynamics is that if U exceed an a/c's MMO (eg by reaching Mach 1 in a 737), which is a structural limitation, chances are it will fall into pieces. In this case the a/c was still in one piece before hitting the ground, so I would say it didn't go supersonic.
Even @ 50,000fpm (about 474 kt) in an absolute vertical descent, we would B far from the spd of sound @ FL200 (613kt) & obviously even further from the spd of sound @ lower levels.
MF