Face it people, Ridley is a clever guy, making movies for a lifetime, used to fielding questions from the press, and he's 80 years old.
In that context, when youre getting the same questions 10 times a day from different outlets, who *wouldn't* have a little fun and tell each one something different?
Edit - just to be clear, what I'm saying in all seriousness is Ridley might make another, it might be in only 14 months ... but it's unlikely, and I wouldn't believe what he says now. Not because he's lying, but because he's spitballing, and thinking aloud, as he does.
People shouldn't take what he says as being fact, until the movie actually starts pre production. Then and only then will we know, huh, he is making a new alien film