If the Disc developed a 'circumferential fracture' of the Drive Arm, a la Miami, it very well could have separated from the remnants of the Arm to remain in the case while rotating independently and progressively more elliptically, scrubbing off its blades whilst gaining rapid deviations in planar rotation. At some point, the energy is greater than the thin case's ability to contain it, and it divides itself into the requisite three piece 'suite', to exit. If the Disc separated, but was contained in some elliptical path within the case, it becomes clear the generation of "fold back" of the Arm remnants on the Wheel's failure signature.
At 7000 RPM, sufficient energy is available to accomplish the Burst, an overspeed is not necessarily required.
So it becomes ever more telling; are the reads on the list accurate?
The 'Rigid Coupling' is accused of failure (ATSB), however, it may have failed in the respect of allowing the AftShaft to wander aft, not in losing contact, other than axial, and that sufficient to keep the shaft operating as a unit. (AD's) IMO.