If "stuff on rocks" burns for only 6sec and if the fireball is 1 action, I don't understand why the characters in the area of effect should take damage over time and/or while moving on the surface after he already had the classic D&D fireball damages.
Not sure we already see another fireball that's not throw on explosives but I'm not sure it's only about the barrels.
As I said, just look at the fire arrow a few seconds later. This arrow also ignite the dust on rocks, giving extra damages over time/on movement.
