From The Register

As expected. The description is that of a light metal fire, where the burning light metal strips oxygen from the electrolyte in the batteries, and anywhere else it can.

This is a very aggressive fire (reaction). If there is any material with oxygen in its composition (and there nearly always is) with heat as the ignition, the light metal can strip the oxygen from the material.

For example: Lithium can reduce Aluminum Oxide to Aluminum, or steal the oxygen from the glue used in the composites of the airplane.

There is no danger to passengers from the fire, none at all. (Note: falls are not fatal. Only sudden stops) so the passengers would not be killed by the fire. Just the sudden stop.

Added on reflection: Boeing’s apparent unwillingness to look at an alternative (even a temporary one) is a testament to bad management.