Cylindrical Li-ion cells, like those part of an 18650 battery, literally hold their chemistry under pressure, and a thermal runaway will normally cause the vents in the cell to release the pressure.
Unlike what you seem to be implying, lion cells are NOT "under pressure" in normal use. They are at atmospheric pressure. The vent is specifically so in case of abnormal conditions, it allows any pressure to escape instead of causing the cell to explode.
result of the release of pressure combined with the reactive nature of now exposed lithium
In a lion cell, there is no "exposed lithium" in the metallic, highly-reactive sense. It's a solution of lithium ions in an electrolyte, whose only real hazard is its flammability.
I believe I know the flashlight forums you refer to, and the incidents of HF poisoning too. Those were lithium primary cells, which actually contain significant quantities of metallic lithium and are definitely more reactive than lion. Even the MSDS you refer to says "in spite of their name, these batteries do not contain any lithium metal".
The MSDS is also the easiest way to scare yourself into thinking everything is highly dangerous. For example, look at the ones for sodium chloride. By the way, I would not trust that one so much as it gives a boiling point for ethyl acetate, a liquid at room temperature, of -84C. Similarly, dimethyl carbonate is listed as boiling at +4C, when that's its melting point. Perhaps that's where your impression that the cells are normally under pressure came from?
Likewise, I recommend looking at the data on the electrolyte solvents:
Unlike what you seem to be implying, lion cells are NOT "under pressure" in normal use. They are at atmospheric pressure. The vent is specifically so in case of abnormal conditions, it allows any pressure to escape instead of causing the cell to explode.
result of the release of pressure combined with the reactive nature of now exposed lithium
In a lion cell, there is no "exposed lithium" in the metallic, highly-reactive sense. It's a solution of lithium ions in an electrolyte, whose only real hazard is its flammability.
I believe I know the flashlight forums you refer to, and the incidents of HF poisoning too. Those were lithium primary cells, which actually contain significant quantities of metallic lithium and are definitely more reactive than lion. Even the MSDS you refer to says "in spite of their name, these batteries do not contain any lithium metal".
The MSDS is also the easiest way to scare yourself into thinking everything is highly dangerous. For example, look at the ones for sodium chloride. By the way, I would not trust that one so much as it gives a boiling point for ethyl acetate, a liquid at room temperature, of -84C. Similarly, dimethyl carbonate is listed as boiling at +4C, when that's its melting point. Perhaps that's where your impression that the cells are normally under pressure came from?
Likewise, I recommend looking at the data on the electrolyte solvents:
https://en.wikipedia.org/wiki/Ethyl_acetate
https://en.wikipedia.org/wiki/Diethyl_carbonate
https://en.wikipedia.org/wiki/Dimethyl_carbonate
Not totally harmless, but not insanely toxic either.