I decided to follow up on Erudite's experiment, and so I added the same page multiple times and got the same author every time. This shows that they don't just randomly pick a page, but it doesn't neccesarily make it reliable. I then did the same thing he did and entered different passages, and instead H.P. Lovecraft. Those are the only two authors who came up.
So what I've come to believe is that this analyzer simply scans your writing for certain themes and styles that other writers use, and they give you the name of the author the passage most resembles. I'd say it's somewhat accurate, seeing as I entered at least 12 passages and got the same 2 authors every time, but I still wouldn't trust it 100%