Definition of thermal runaway
To an engineer, thermal runaway is when temperature elevation triggers something with the effect of releasing energy, further increasing temperature, leading to a self-sustaining positive feedback bringing the temperature outside of the design range.
In the context of a battery,
- the something that's triggered is liberation of the energy stored in electrochemical form, as heat and/or the release/expansion of gazes (this can contribute to the positive feedback by both mechanical and thermal means);
- thermal runaway can be split into two categories: within a cell, or propagating from a cell to adjacent cells; the NTSB report of the Logan incident suggests both categories of thermal runaway occurred (7 out of 8 cells had short circuited, strongly suggesting they released most of their energy).
Prohibition of thermal runaway in aviation Li-ion batteries has been worded as: "Li-ion batteries must be designed to preclude the occurrence of self-sustaining, uncontrolled increases in temperature or pressure", with no reference to the damage such event might cause.
Restricting thermal runaway to an unwanted release of energy at a level endangering the plane would be wreaking havoc with that engineering definition. I see it as a miserable attempt to deny the failure of Boeing's initial analysis that the battery as a whole was safe from thermal runaway.