[simonh]

That assumes the potentially chaotically turbulent, expanding blast of exhaust gas propelling these rocks only imparts a single instantaneous linear impulse. It's possible some rock fragments might be kicked up from the surface, and then accelerated laterally away from the landing site by the gas cloud.

[Diederich]

Perhaps? Delta-v from the surface of the moon to orbit is about 1.6 km/second, though debris from a landing would be approximately instantaneous thrust, which doesn't allow insertion into an orbit.