The photo of the recovered disc segment (1/3 sector) strongly suggests it is NOT a rim-originated failure, but rather starting at the bore and extremely rapidly progressing outboard. Such is characteristic of an overspeed burst, and the metal splatter from the 'false bearing' is supportive of this.
The root cause of the rotor burst is the fact that after the IPT disc decoupled from the IPC, the IPT blades remained more-or-less intact in the disc, and continued to convert N3 gas stream energy into torque to accelerate the disc to destruction. Note this sequence is different from the RB211 failure on QF74, 31 Aug., where the blades were destroyed/removed either by blade root failure or by clashing with the LPT nozzle ring.
Don't get me wrong; Oil fire is nasty - you DO NOT want that blowtorch playing on bearings and shafting. But it should not precipitate an overspeed to the point of a rotor burst!