From that linked article and video:
“We’ve seen batteries balloon in the past in iPhones and other smartphones, and again, it’s almost always due to damage resulting from an impact of some kind.”
It happens...it happened to me, not the bulging but overheating and melting. It didn’t cause a fire, but burned the phone and the leather case in three areas and self-extinguished. I’m pretty sure it wasn’t even charging when it happened. No damage to the phone, no third-party repairs (or any repairs for that matter), and only Apple chargers were used when charging.