"Open to interpretation" means the same thing as "undefined behavior in the C programming language": i.e. only overly pedantic programmer nerds stick to the assertion that it could mean "anything."

When a work is said to be open to interpretation, it means that the author has their expressed opinion, but that it may also suggest other things about the author that they are not willing to admit about themselves.

For example, no interpretation of Orson Scott Card's "Ender's Game" will draw the conclusion that Card is a fan of Jurassic Park, or that he takes his tea with three sugars, or any such thing. There's just no evidence in the work to suggest one way or the other. But you could take his assertion that the book is about camaraderie and the struggle of good versus evil at face value, or you could take the themes of preemptive, hyperviolence he uses to read between the lines and realize that Orson Scott Card is actually a fascist neoconservative.

That's what "open to interpretation" means. A particular work does not exist separate from the person who created it, and people are not reliable narrators on their motivations.