Boeing is trying some damage control redefining "thermal runaway" and "fire".
My understanding of the reaction that occurs during thermal runaway (which, that's undisputed, did occur at the cell and battery level) involves recombination of the oxygen originally weakly trapped in the Lithium Cobalt Oxide cathode, with the anode's material (is that graphitic carbon ?). That generates heat (unless I err, chemically in addition of any electrical effect), and smoke, and propagates. Boeing even puts a "fire hazard" sticker on the stuff.
Draw your own conclusions on if thermal runaway at the cell or battery level qualifies as (contained) fire onboard.
That said, we can have some trust that what Boeing proposes greatly reduces the risk that thermal runaway of a cell degenerate into thermal runaway at the battery level; and trust that thermal runaway at the cell or even battery level, if it occurred, will no longer by itself endanger the plane. Only the combination of these events with some other event where the battery is needed could cause a disaster.
Thus questions now are
1) How likely does cell/battery failure remains ?
2) What if a battery no longer supplies adequate power ? There are variants: open or short circuit, degraded voltage source..
3) Are there other oversight of failure modes elsewhere in that plane?
Answering 1) is important, and depends a lot on what the root cause was in the first place. Unfortunately, here we can only speculate. I guess Boeing, and I hope the NTSB/JTSB, have much more information by analyzing the data available on the battery's history, and the other batteries. My guess is they'll get to the bottom of it.
2) Is about airplane safety, and well understood by the specialists (not me).
3) Well, I hope both Boeing and certification authorities are busy at it.