A Rocky Start
The rocks stopped being able to reach the fleeing transport and mechs after passing roughly a kilometer of distance. Simple gravity prevented the rocks from flying any further, though they rolled forward at a considerable distance even when they landed.
Ves immediately recognized that the rocks had deliberately been chiseled into a rounded shape to facilitate the rolling motion.
Even though launching rocks seemed to be an incredibly primitive form of attack, the possible damage they could inflict was no joke! Their transport would have suffered a huge dent if hit!
The main problem with relying on throwing boulders was that they weren't very accurate. While the rocks launched under standard gravity conditions inside the city, once the projectiles left the antigrav field that surrounded the city, they almost literally started dropping like rocks.
That limited their eventual range to a kilometer away at best.