In the case of a sealed container, you would be correct. However, the current 787 container can vent (and did so in the case of JA804A) and the new container would include active venting to the outside of the plane.
If the next event happens as JAL BOSTON did, there would be a grave danger of being drenched in flaming electrolyte, bits of battery, and extremely hot vapor?
Would not want to be a rampie, or a crew doing walkaround.