From the drawing, I note a build up of the LPT vanes platform at the area that makes contact first with IPT should it drift aftward. The portion of the IPT that makes contact with this strengthened area is the blade root circle. This in anticipation of wheel release, the design is meant to remove the airfoils in just such an emergency as this explosive burst.
As the design consideration, it demonstrates an improvement in concept, an anticipated mitigation. I suggest that it worked; as bad as this incident appears, if IPT retains its blades any longer than apparent here, the a/ c may have been lost.