I don't believe that Noomi and fassbender had already signed. I think thats BS. Plus wouldnt they have that in their WIKI page? if this is late breaking news, then we will see by end of day today.
That type of deal usually follows an extraordinary for-sight based on world wide earnings for Prometheus and sequal announcements. Plus thats the assumption that the sequel is a continuation of the last film and not a story in itself which i am personally more interested in.