Had to comment. They are shooting fire at a fire dragon.
And? The reason why a lot of creatures in fantastical settings are resistant to something is a) magic b) the creature itself is of the same essence (fire vs fire), or combination of both. I guess i don't need to tell you that this shit is absolutely arbitrary. Meaning, the fact that Dragon has some kind of fire gland or something doesn't mean the rest of his body is made of different material than rest of the world (thus, being resistant to fire, while everything else change/is destroyed when exposed to high temperature).
So the conclusion is, there's nothing wrong with killing dragon spitting fire with fire, unless it contradicts the lore, which it doesn't. You try too hard to be nitpicky, yet your post is retarded.