The movie doesn't address this inconsistency. It's likely a creative liberty taken for dramatic effect, as fire is a powerful symbol of human ingenuity and dominance.
Here are some possible explanations that the writers might have considered (though none were actually presented in the movie):
* Alien fire technology: Perhaps the Na'vi have developed their own unique way of creating fire, perhaps using a different chemical reaction or source of energy.
* Fire is not essential: The scene where Jake fights with fire might be symbolic, representing his strength and resolve, rather than a literal use of fire.
* Plot oversight: It's possible that the writers simply didn't consider this detail, or decided to prioritize the story's impact over scientific accuracy.
Regardless of the reason, the inconsistency highlights how even in fictional worlds, scientific accuracy can sometimes be sacrificed for storytelling purposes.