While the steel box got all the attention, they also redesigned the battery. Basically they increased the space between cells and added more insulation such that, if a cell faulted and started to overheat it wouldn't propagate to the surrounding cells. There were other changes as part of the fix but I don't recall details.
Since return to service, there has been one report of a battery problem where a single cell overheated, but it didn't propagate.