Much like testing for certain medical diseases, sometimes you can only determine a cause by exclusion.
- A phone that is turned off is consuming no power, so the failure cannot plausibly be caused by an excessive rate of discharge or by external heat (e.g. being too close to a hot GPU).
- A phone that is not charging is adding no power to the pack, so the failure is probably not caused by an excessive rate of charge or by overcharging.
- Multiple battery manufacturers use different battery chemistry and different designs, so the failure cannot plausibly be caused by dendrites or other similar failures. Also, the failures don't occur with those same batteries in other devices, which eliminates the batteries themselves as a likely cause.
When you eliminate the impossible, what remains are failures that can occur even with a battery that is neither charging nor discharging. The most likely causes, then, involve some form of physical damage.
LiPo packs change size during normal charging and discharging just a bit. That's why there are tolerances build into the design. With insufficient tolerances, bad things happen (TM), and even if the tolerances are sufficient to avoid self-puncturing at their maximum size, it is possible that flexing the case in just the right way while the pack is maximally swollen could still puncture the pack. So this is at least a plausible explanation, whereas most other theories aren't.
With that said, even if we assume that these folks are correct, it does not absolve other aspects of the design. Not all failures have only a single root cause. For example, IIRC, overcharging a LiPo pack can cause unusual levels of battery expansion from hydrogen buildup, which when combined with normal levels of flexing in a case that has insufficient tolerances, would result in the pack perforating and venting with flame.