They had to have known they'd be fighting aliens. The elder had a burn mark in his helmet, so it should've been apparent that they knew what they were going down for.
History has shown them that a whaling station was positioned very close to the underground pyramid. Now maybe the human race has been watched for a while by this group of predators or maybe not. If they haven't, it's likely the predators thought that a new population took over the activity at the station and continued on to be in wonderment of what happened to the old inhabitants. What we do know is that in 1904, the predators were not so well capeable of keeping the aliens underground either as we see one just to the left of the burly chum whom I so lovingly shall call Bjoern (because he looks like a Bjoern). It's possible the predators were expecting the next round of humans to be only slightly more advanced than our pal Bjoern, and when you consider the completely useless drill team, they guessed right. It wasn't until those slimy corporate guys started in on their treasure pillaging that the predators dropped load in their armor and hit the panic button.