LiCoO2 is mature technology. These cells are derated to 4.025V EOC and ultra reliable.
There is too much damage to the cells. It is improbable, that the cause of the Primary failure can be conclusively established by forensics.
Further analysis of the electrical system and of reported wiring errors will provide clues.
Now if Boeing wants to contain the fire, and hope it's a 1:1'000'000 thing, this leads me to believe they're leaning towards field failure as a probable cause?
I doubt that. They just want to make sure.