The accuracy of the ILS system is probably not open to doubt.
Therefore the next question is possibly how accurate the automatics if used are at holding the LLZ & GP, or if hand flown the same accuracy of the PF.
Given the eye witness account of the steep RH bank the alternatives are a failure which caused this or a control input. It's unlikely that a control input would be made to flay away from the centreline so the theory off being off the centreline must be the greater likelihood if, as stated by the investigators, the aircraft was operating normally.