Something is only a prequel if it's released after the previous book(or movie, or whatever) yet takes place chronologically before it.
The Hobbit book was released first, so it's not a prequel to anything. It's simply a book. The LotR trilogy was released after The Hobbit, ergo it's a sequel.
Since the LotR movies were created and released first, technically The Hobbit movie trilogy is a prequel.
Your definition isn't correct. Any story that takes place before another is always either the prologue, or a prequel, regardless of what Hollywood makes of it. The events of "The Hobbit" book take place before the LOTR Trilogy books, and therefore, qualify under that definition. The movies just happened to be made and released in the opposite order due to Peter Jackson being more interested in the LOTR stories first.
You're just so used to the movie version of the "prequel" definition that you think that's the only way to define a story that takes place before a major one, and that's not true.
You could say that the film is a prequel to LOTR but you could not say that the Hobbit book is a prequel to the LOTR books. It is rather simply the book that was written and published first. To use the term "prequel" implies that it was published afterward.
However when Tolkein wrote the book it was a standalone children's book. It was never planned to be connected to any other story. Then it was retrofitted into a prequel to the Lord of the Rings by the author himself.