No, that hasn't been established - at all.
Okay, there might be a chestburster in the film. So what? It's set in the same universe and Lindelof has suggested that Alien fans will get their "encore". That means we may get some facehugger/chestburster action. That doesn't mean that they are lying about the film not being another Alien film. They are trying to do something different here and for them that's enough to be able to say this isn't an Alien prequel.
If this was a "fully fledged Alien film" then you would expect it to be about the same thing the other films were about: facehuggers, chestbursters and xenomorphs. That's what people would expect from an Alien film. They haven't called the film Alien because that's obviously not what the film is about.
So its set in the same universe, with the same elements (Weyland / humans / SpaceJockeys + ship / Xenomorphs) - but its not an Alien film...
Personally, I don't think Scott is that clever, or stupid.
Its going to give us an introduction to the Xeno's, and provide a bit of a genesis story for them. Just because every second scene may not involve one, doesn't mean the premise of the film does not focus on their creation etc