I agree with you, but I think there are a lot of LW people who didn't really like the meta-ethics sequence or liked it but got something odd out of it and who basically think that most of what they value comes from genetic-evolutionary pressures (the aliens in your scenario). Luke's post is very important for them if not for the rest of us who are more interested in where we're getting our notion of 'right' from if not entirely from the aliens.