SM consults with the people currently in charge of the Alien franchise license (outside of the films) -- comics, novels, games, etc. It's not so much that SM's word is law, as it is that the people Fox have entrusted with the franchise between films use his knowledge and expertise to keep shit together.
Basically, SM's word is trusted by those that Fox trust to keep the franchise together between films, ergo, SM's word is a pretty damn good indication of what Fox is happy to declare official canon.