In my mind there are 3 key elements in the AAIB report:
1) There are no other aircraft systems in this vicinity which, with the aircraft unpowered, contain stored energy capable of initiating a fire in the area of heat damage
2) It is not clear however whether the combustion in the area of the ELT was initiated by the relase of energy in the batteries or by an external machanism such as an electrical short
3) In the case of a short, the same batteries could provide the energy for an ingnition and suffer damage in a subsequent fire
My interpretation of these words is that the ELT is a suspect but is far from being confirmed as the guilty party. In simple terms, the guy was there, he had a gun, but we can't find any bullets.