^I think it's safe to say in this instance "not a prequel" means not a prequel in terms of plot and narrative. This is not the story of a crew that did the exact same thing as the crew of the Nostromo, only a few years prior, nor is it a tale of young Ripley struggling to be a single mum space trucker. It's a new story that takes place earlier in the timeline in the same continuity. The only possible link being that derelict, or perhaps just the race that built it. Actually, I think according to Scott, Blade Runner is also meant to take place in the same continuity.
Ah, Alien yes but not as we know it. He said no Xenomorph. The Xenomorph might already have been a variation of another race if we ignore the PVsA films. So what we see here might be a pre pre version something we don't know is Alien at first.
IF we ignore Alien vs Predator? IF!?!!? You say that as if there could possibly be a sane alternative to ignoring them.
Anyway...From what I've always taken to be Ridley's take on things, the "xenomorphs" (a James Cameron invented moniker IIRC) were just bio-mechanical weapons and that derelict was a "battle wagon" for deploying them. I think he's even said that the space jockey isn't a life-form in and of itself but another purpose built bio-mechanoid, which probably means the race that created them are something else entierly.
In that recent interview he mentions 'Chariots of the Gods' which combined with that shot of a huge human looking (but still Giger-esq) face suggests that whatever these things were, they've been to Earth before and possibly had a hand in our own evolution. An old story yes, but then Scott is a big fan of 2001: A Space Odyssey