...I think I mostly agree? I mean, canon vs non-canon is a useful distinction in some contexts, but it's not necessarily well-defined in the general case. There's a meaningful sense in which all interpretations are equally valid (and another, also meaningful sense in which the author's interpretation is "canon" and privileged in some way).
If my interpretation of The Lord of the Rings is that whenever we weren't looking Sam and Frodo are passionately making out, then in what way can that be false? It's not like there was a real Frodo who either was or wasn't making out out with Sam. There is no Frodo. He only exists in the mind of people reading the book/watching the movie. Ergo, if in my mind he was making out with Sam, then the version of Frodo that lives in my mind makes out with Sam. The end.
The version of Frodo that lived in Tolkien's mind probably didn't, which means that I shouldn't expect to find a line in the epilogue that reads "And then Sam left his wife and children and moved with Frodo to Rohan, where same-sex marriage is legal". In that sense, "canon" is privileged, because it can be used to predict what will actually show up in the page. But it's not any more true or real than any other interpretation, because no interpretation is true or real at all. Fiction is things that didn't actually happen, hence the name.