The way I see it, a prequel is specifically designed to set up the events in the films it's, uh, prequelling. I could be wrong, but this is just a movie set in the same universe, before the events of Alien, but has nothing to do with the Nostromo or xenomorphs.
And yet, it seems like we see the crash of the Derelict, which I thought was as a result of an infestation of xenomorphs, so maybe I don't know what the heck I'm talking about. :/