This means that the Honeywell supplied ELT was the source of the fire which affected the composite structure and interior insulation as well as blackened and peeling the paint externally. There was damage to the composite structure. The firefighters tried using handheld halon extinguishers which were not effective then resorted to water hoses which put out the fire. The ELT was effectively destroyed and the AAIB also said that there we no other aircraft systems in the vicinity of the ELT "which, with the aircraft unpowered, contain stored energy capable of initiating a fire in the area of the heat damage." The AAIB is not certain if the combustion was initiated by a release of energy within the batteries or an external source such as an electrical short.
There are two recommendations:
1) The FAA orders the inerting of the Honeywell ELT until it can be fixed.
2) The FAA initiates a safety review of the lithium powered ELT in other aircraft types and mandate appropriate fixes.
Essentially this clears Boeing and the electrical architecture but it would be paramount to understanding the source of the combustion. The Honeywell ELT is used in other aircraft types and have been certified for use in commercial aircraft since 2005.
The AAIB also said that there typically isn't any fire detection or suppression equipment in the space where the fire took place and if it had occurred during flight could have posed a "significant safety concern and raise challenges for the cabin crew in tackling the resulting fire." The firefighters had to rip down the ceiling panels and overhead luggage bins to access the area of the fire.
With inerting the ELTs, the risk of a fire is taken away. Boeing will still have to determine if they can economically repair ZA261 and return it to safe and normal service. This will be a major challenge for Boeing's engineers. You can read the full AAIB interim report: