Marcus is trying to kill a swarm of bugs. A good amount of fire seemed like the right idea. Given that Savanna fires generally move at speeds up to about 7 kph, the trucks are safe from it.
Note that Marcus fired a round BETWEEN the trucks on one side and the bugs on the other. Not directly in the middle of the trucks. He's not that big of a moron, even as stressed out as he is now.
If it was a normal savanna and normal bugs you'd be right. But they aren't. Obviously the entire mission was going to lead to a trap, but the trap is bigger than just the camp.