The Covenant also originally had weapons that didn't make it into the game. There were once several different kinds of rocket launchers, some anti-tank, some anti-personnel, and some anti-aircraft. It's a good guess that this one didn't make the cut when it was decided to make all Covenant weapons non-projectile; the only similar weapon in Halo 1 as shipped is the Fuel Rod Gun, used by Spec Ops grunts and molded into the arms of Hunters.