Bearfoil
I think a better depiction of the 900 architecture is presented in 26/11/10 Post#1375 by No-Hoper. I think it has changed somewhat from the RB-211 526G as to the IP rigid coupling. If you look at this 900 cross section, the rigid coupling (color-pink) is just aft of the number 4 bearing. It is a axial spline coupling, maybe helical in actual design. Just slightly forward of this is the rigid coupling (blue-color) for the Fan - LPT shaft. Upon disconnect, it is the actual spline at the end of the shaft you were questioning in the photo of the pulled LPT module.
As a matter of interest, the IP shaft material would be a high strength steel selected probably based on tensile strength (the higher the better) to handle torque/twisting of the shaft. Typical materials are maraging steel or equivalent. It does not have very high temperature capability nor superior wear resistance, so the splines are probably coated to overcome this deficiency. So it is this IP rigid coupling which is concerning for spline wear as rearward slippage would move the IP rotor towards the stage 1 LPT nozzle ring. Not having any photos of the engine forward of the LPT module, it is difficult to know what exactly permitted the IPT rotor to move rearward. Did it pull out of the rigid coupling or did it fail (softening) because of the fire beneath the IPT rotor where the bearings are located, or without oil, did the shaft get scored and then just broke.
Nevertheless, I do think there is a strong possibility, that spline wear contributed through unexpected vibration or harmonics to the failure of the stub pipe, sooner rather than later.
Just some information & thoughts on my part, cheers!
Turbine D