No I don't think it's the book / site at all, this title as "one above senior" predates the book by a very long time; I've been looking at job postings and it is pervasive and not at companies that just started using it that way within the last couple years.

The "it's a neutral word so it should be a neutral level" idea is clever but I don't think that's where this word usage comes from. I think it seems like it might have something in common with titles in media companies. What is a "staff writer"? Is that a fairly senior position? I dunno.