The thing is, there's both too little and too much information about an author's intent to be able to really discuss it. Too little because there's never enough real evidence about someone's state of mind, too much because what there is isn't necessarily reliable. So "authorial intent doesn't matter" is a shorthand way of saying "discussions of the author's state of mind are not as likely to bear interesting critical fruit as discussions of the text itself." In terms of what the author says, it certainly makes sense to discuss the author's interpretation, but it is still best to argue for or against that interpretation based on evidence within the text.