Also, as 10ebbor10 said, this system was searched, and any space-based vehicles or any air-based vehicles would be easy to find.
Then there's no way that we would be attacked, because you can't exactly march an army to a different planet.
You misunderstood, they would've fled to the moons from the planets below. They could attack us.
And if they can get from those moons to us, they must have spacecraft.
And if they have spacecraft and want to attack us, they can simply threaten to/drop rocks and go from there.
Heck, having a centrally-organized-and-planned military would be a drawback at this point, because it would give them an obvious target.
((And that's assuming they'd have reasons to attack us in the first place...))
The bombardment was 80 years ago. The spacecraft we harvested could've been the one they used to get here. It is easier to steal food than to grow it yourself.
Wait, I thought we were harvesting our own spacecraft...
Nope, that was a derelict that was unburied by a sandstorm.
Ah.
Still, if they're living on this planet, they're at a level where they can't be detected from space. So, no radio, nothing used for light at night, possibly no electronics, no cities or other settlements aboveground...maybe other "nothings" depending on what sensors are available. Also:
This system was bombarded during the seperation war 80 years ago. The alliance won, and has been in charge ever since. They have been quite thourough.
This implies that they know about just about anyone who may live in this system. As no one was mentioned, no one probably exists.
