Unpressurized cargo holds date back to a time when we had to slacken guitar strings to haul in cargo. Commercial modern Jet aircraft pressurize to around 8-10k’ ASL.
I recently drove with my Quad from San Diego up to White Mountains of Arizona around 6500ft ASL and none of 6qty 2.2Ah bricks exhibited any noticeable puffing. However, they’re in very good shape.
While I have flown with RC Lipo unlike the person referenced in OP I’ve also followed all applicable TSA guidelines. I never try to hide them and I would never, ever, bring any puffers on board AC. Puffers would never be a good idea to haul anywhere.
“What might you do with the carry on bag if it starts smoking?"
Probably same thing if it were a laptop, tablet, phone, power brick, tool battery, etc., that started smoking in carry on bag. Quickly evaluate the situation, notify flight crew and follow their directions. If reasonably possible maybe attempt to move it to a lavatory?