Yes, Alien being missing is an odd sign, especially as it has 99%. (IIRC it used to be the only 100% film, until some critic posted a new review saying it hadn’t aged well)
Alien is there (position 56).
Dang it I both scanned and searched the page, and safari browser said no text results.
Their "adjusted score" means that films with more ratings are rated higher. Older films have far fewer ratings by critics, same with smaller films.