Yeah, IDK any number of things could have happened on that incident. From a grease fire, to a tank hooked up wrong. Kinda hard to do a grilltopsy from a couple photos and an aftermath. Good speculations though. And yeah the smaller Q's are designed to or have the ability to run on small bottles BUT let's not forget Weber themselves sells an adapter and IIRC on some even includes it to run the same grill on a larger tank.
So, can't necessarily point to that as a "fault" of an operator