^I'm guessing they're trying to downplay the connection in an effort to avoid having viewers expect something that they're not going to get. When you ask someone about the Alien franchise, how often do they think of something other than the xenomorph? I think it's been said multiple times that there won't be a xenomorph in the movie, so I'll bet they're trying to downplay connection in an effort to avoid luring people in on the false hope that they'll see the xenomorph. Yes, the movies are all in the same universe but by coming out and declaring this an Alien prequel, they'll send the expectations in an entirely different direction.
You're overthinking it.
It's a movie. It's a science fiction movie. It's a science fiction movie made by Ridley Scott.
Also, nobody but someone who doesn't get Alien movies talks about a xenomorph.