I have always interpreted "moves" in this context to relate to straight line acceleration, like when somebody says a sports car "really moves" they mean it's got quick straight-line acceleration.

Immediately after this quote Ford (or maybe Zaphod?) says something about how he had been passed by some ship at great speed, and shortly after it crashed into a planet or something. I can't remember off the top of my head. But that seems to support the interpretation that it can move fast but can't steer.