The wild and crazy parts of Scientology (Xenu, souls of dead aliens, etc.) do not appear in Scientology’s books, course materials or other documents initially. They’re supposed to be a big secret, which you only find out about if you reach the stage called “Operating Thetan III” – commonly referred to as “OT3″.
Only about 10% of Scientologists ever get that far, and they are forbidden from discussing the course material with anybody – so it’s very easy for Scientology to wheel out a selection of people who can honestly say they’ve “never heard anything about aliens”.
If you want to learn the basics of the Xenu/OT3 story, someone has produced a great video which combines the South Park explanation of the story with L Ron Hubbard telling it in his own voice.
The veracity of the leaked documents has been proven repeatedly, due to Scientology’s lawyers using US and International copyright law to try to have them removed from the internet. A major stash of documents relating to all the OT-levels is still available on Wikileaks.
A number of ex-Scientologists have also confirmed that the leaked documents are accurate and represent what they were taught / given on the OT3 course.