This is not, I repeat, this is not a prequel to Alien. It is a movie that tkaes place before Alien. There's a huge gap between the ending of Prometheus and the beginning of Alien.
Hang on - how is this not
a prequel to Alien? It takes place in the same universe (or, at least, it appears to), it references different aspects and set pieces that were in Alien (space jockey, derelict ship, etc.) Time gap? So what? How many years had taken place between Enterprise and TOS Trek? 80+ years? Or between TOS and TNG? Over 100? I haven't seen it yet myself but the logic here doesn't track.