That iPhone 6s was in to a 3rd party service shop for repair due to an old, failing battery. The tech had removed a cracked screen and was testing the old battery, also a previous 3rd party replacement as I understand it, when it exploded. The standard accepted failure rate on lithium ion batteries is one in 8 million. . . Although usually they fail not quite so spectacularly as this one.
And of course, that failure rate is taking all of them across the market and around the world into consideration. It lumps the most cheaply-produced, dangerous-from-the-factory garbage with the premium OEM-spec batteries.
The failure rate also includes those that are abused, neglected, and generally treated in a way that would make anything fail.