Failure to handle short circuit is significant
What I find more interestingis the failure of the pack to handle a short circuit safely. The use of a PTC for this is very common but the variability of trigger point and it's dependance on environmental conditions make it tricky to be sure it will protect and quite likely that short circuit protection could be successfully tested yet a catastrophic failure is still possible. The normal way this is tested is simply to introduce a dead short and check it is handled adequately. resistive or intermittant short circuits which are critical to how the battery pack could fail despite PTC protection are not tested.
A battery short circuit is a very obvious failure mode which any design should handle as this battery pack was clearly intended to.
We have designed many medical devices with simlar packs and PTC protection although we usually have an additional safety device on the battery so it is a concern that such a pack failed in this way even if we have another layer of protection beyond that used in this case.