Maybe. More likely it was some type of launching/pushing mechanism. It just looked, and acted, gravity fed. It's one of those things where the more I think about it the more it doesn't play right. It didn't seem odd at all when I saw the scene in the film - just later when it hit me there's no gravity to pull the bombs out and "fall" onto the ship. It's no "deal breaker," just a bit odd...The way I see it is that all the ships have artificial gravity in their interiors. As long as the bombs were inside the ship, they would fall normally. Momentum would do the rest once the bombs exited the ship.
You wouldn't run the thrusting engines just to provide power. That's incredibly wasteful. You'd have generators for that which should require minimal fuel compared to the thrusters.They would still need the engines running which I assume power the rest of the ship. So they would still be burning fuel even if they were up to speed.