I think it's more simple than that. It's not that this movie is a reboot or that a sequel would necessarily ignore the events of the "Alien" saga. It's just that a sequel would be about different people doing different things. Ripley and the Nostromo (and the Alien sequels) would still happen, but they would be irrelevant to the new story being told. It's a big universe; lots of things are allowed to happen without being related to each other.