Aliasalpha: Well unless either of the ships mentioned is the one that crashes on LV426, its not so much a prequel as it is another film set in the same universe. If it doesn't set up story threads that are resolved in chronologically later stories (why the LV426 ship crashed or the genesis of the aliens) then it won't really be directly connected. It's more likely to be a simple slasher movie on a space ship again.
For example Star Trek 3 & Star Trek 4 were definitely prequel/sequel because 4 used story threads established in the previous film but Star Trek Generations & Star Trek First Contact were 2 films set in the Star Trek universe because the stories were completely unrelated
I think that by 'prequel' they simply mean it's set at a time in which the alien species still existed. If it's going to involve humans at all then it's most likely going to be a Year One style retelling.