For personal interest, I went and checked, and while I was unable to verify this, apparently the first use of the term in this sort of context was in reference to Sherlock Holmes. It referred to the stories specifically written by Arthur Conan Doyle, while all other authors using the character are 'non-canon.' If true, this means that originally, it wasn't about things which are confirmed to exist at all, it was about who had written them (Which makes sense, given the history of the word itself, no more on that here.) Following that logic, Frank Herbert's Dune is canon, while everything Brian Herbert did is not. Which I am ok with, incidentally.
This would get a little muddled though when you run into things like Star Trek, which has had numerous writers through its history, or 40k, which is an ascended tabletop game with multiple creators. Star Trek was created by just one person - Gene Rodenberry - but plenty of Star Trek that has been made without his input is still considered canon.