So the electrical short igniting the fuel cell is a certainty?
That the fuel cell ignited is a certainty. The exact cause of the spark that ignited it is not. Along with electrical shorts (which are certainly the leading contender) they looked at a lot of other causes, for instance something mechanical in the pumps scraping up a spark, or something caused by the very warm AC units. They were able to come up with a number of theories but none of them easily built into a refutable, testable hypothesis. So they left that question open.
This occasionally does happen, that there are details, even significant ones, that can't be pinned down. In the Lauda Air 767 that had an uncommanded thrust reverser deployment over Thailand, they were unable to determine if it was a hardware or software failure -- so they changed both. Here with 800 they took a similar all-points approach to making sure there could be no repeat.
I must say, Mr Valance, for someone who was so famously shot in song and on screen, you're looking mighty healthy.
d.o.l.
Criminal Number 18F