On the plus side, planetary distances are so large you know for a long time that someone is coming to your planet. So you have time to prepare a reception.
Here they would not be defending other terrestrial intruders but rather space invaders from earth, so they'd be reasonably sure where to look out for coming troublemakers. On the other hand, they could be in a precarious position if their habitat isn't terraformed (not needing intensive energy to keep things habitable, thus a weak point in sustainability).