I think there is a tendency to group Terminator along with Alien and Predator because they all have a certain real-world feel about them. It isn't so much about them all being part of the same fictional universe, but rather each of them being congruous with the universe we actually live in. That said, as of AVP they can't really be seen to exist in the same continuum, and if you're going on the alternate timeline thing then you're resorting to a separate universe anyway, so what's the point?