The captain of the Hyperion specifically states that he was already IN
hyperspace, en route to another system, when he was ordered to stop here. His
ship was already in the area. Also, there is a fair amount of time between
the actions on the planet initially, and the arrival of the Hyperion...enough
to know there's some kind of tech there, based on the initial attacks. The
Hyperion continued to monitor reports as they came in.