LandIT
Do we think that containing a momentary explosion caused by the introduction of propane gas, proves anything about an effective containment of the thermal runaway reactions of volatile chemicals released as a consequence of the battery failure they experienced twice already?
I don't think it does.
I am sure neither does the FAA so to assume this was the only test Boeing performed is not logical.
Boeing has really screwed up over outsourcing the risk-sharing and external design parts of that airplane.
I doubt Boeing designed the Ship's and APU batteries for any of their commercial airliners. I expect every one of them had their design and production outsourced to external suppliers.
Blacksheep
The battery is a completely new design, but the containment box ensures that the travelling public - and many uninformed aircrew - will continue to regard the B787 battery as a highly dangerous piece of equipment.
If those changes to the battery stop them from failing, soon enough the traveling public will forget about the containment box just as they have forgotten the nitrogen in the fuel tanks to prevent a fuel-air explosion and the armor in the nacelles to prevent a failed turbine blade from being shot into the fuselage.