That could just mean they took the gutted shells and stuffed them full of Trukk bits while fixing up the turret, though - everything we're told about Looted vehicles is that they are crude, ramshackle, and occasionally as dangerous to their crew and surrounding infantry as they are to the enemy...exactly how the Orks like it. Even WAAAAUGH power can't create parts out of thin air, only make parts they have work better/longer than they should. And there's still what I mentioned before, that if the Zerg killed the vehicles once when they were in pristine condition, they can easily do it again when they're salvaged and crewed by Orks.