I think that talking about the battery pack being inherently unsafe is completely off-topic here. GS Yuasa manufactures batteries for 60 satellites and they haven't had a single failure there
http://www.gsyuasa-lp.com/content/gs-yuasa-lithium-ion-batte...
In aviation people usually very thoroughly figure out what went wrong and why and only then suggest fixes. And the fixes are designed by the people who have access to the complete system.
That's a hilarious observation, and I've upvoted it.
However, batteries (as opposed to capacitors) get their power from redox reactions. If we use a definition of "fire" that includes the rapid redox reaction between liquid phase propellant and solid phase propellant in a hybrid rocket motor in the vacuum of space, that definition would likely also extend to rapid redox reactions between liquid electrolyte and solid electrode in a battery during a particularly fast thermal runaway in the vacuum of space. In other words, under many reasonable definitions of "fire", any sufficiently rapid thermal runaway in a battery would be a fire, despite there not being any air present.
Musk's allegation is that the 787 battery is a poor design, not that it was manufactured incorrectly.
Your point would be a good point if Musk's allegation were that GS Yusa had manufactured the batteries incorrectly or if the satellite battery were of the same design as the 787 battery AND used under similar conditions.
However, my understanding is that the 787 battery is a new design and used under conditions rather different from a satellite.
In aviation people usually very thoroughly figure out what went wrong and why and only then suggest fixes. And the fixes are designed by the people who have access to the complete system.