Given my experience with Hawaiian expertise, the firefighters saying that "the fire was extinguished" is somewhat less than credible.
More likely, they spent an hour or so putting it out but got hungry and left without inspecting the site for any remaining live embers.
Firefighters were called away to other fires around the island. That's from an eyewitness who was in close contact with Lahaina firefighters. Yes, grassfires are tricky.