Re: Mechanical damage
That would be a reasonable assumption, however my experience tells me otherwise
I have seen mobile devices and laptops seriously bent out of shape, or otherwise almost completely destroyed, but the battery, even though it's bent well and truely out of shape, has shown no signs of expansion, venting, or anything indicating that it's suffered a thermal event.
On the flipside of that I've experienced batteries that have suffered a sudden and violent expansion, and/or thermal event without any indicators that the device was in any way prevously damaged.
I'm inclined to agree with the girl in this instance. From the video the phone shows no damage, apart from what would be normal for an expanding battery. The venting and thermal event is likely to have been caused by the battery puncturing as it expanded - shouldn't happen, but it does.
Given that the phone was taken to an Apple store because it wouldn't turn on at all then it would indicate a bettery in a very low state of discharge - this is bad as that's one of the danger points with LiPo cells when it comes to recharging them. That the Apple store staff "got it going again" is no way idicative of a problem resolution - we all know that getting something going again is a long way from solving the underlying problem.
So, my take on this is that it was just a bad battery, that one in a million, the one that happens inevitibly every year.