Authors are full of it though and beyond this*, there's also the matter of changing genre classifications. You have to get quite a long way through fantasy's history after all to find authors who recognised what they wrote as fantasy - do we remove Tolkien and all prior writers from the genre? I don't think anyone would suggest that, so it seems to make sense that the work of authors who have stated they were writing in one genre might be more usefully considered as part of another. McCaffrey's Dragons of Pern series seems an excellent example - given the status of fantasy when she started writing them, I completely understand why she might have considered them sci-fi. Fifty plus years after the fact, the general status of the fantasy genre makes dropping them in it pretty obvious. Ditto how Star Wars is a sci-fi no matter what Lucas might say about it being a fantasy.
And speaking of Pern, if the standard of possible scientific explanation based on undiscovered science for seemingly impossible powers (thus qualifying for sci-fi) is Pern's time-travelling teleporting dragons, then about 33% of the fantasy I read should now be classified as sci-fi including Lord of the Rings, Wheel of Time, Discworld, etc.etc. That's a conservative estimate. Also equally drawn from superstitious and what not elements.
In any case, and I know I'm not the first person to say this, but why to draw a firm line between two genres that are considered hand in hand, that borrow from each other frequently, share a very large sub-genre or two in common, and share many of its most famous authors? I'm about as strongly one genre as can be and yet still recognise there's a large overlap on the venn diagram with a lot of authors choosing to camp there.
*my favourite example, above and beyond JK Rowling trying to claim Harry Potter wasn't fantasy, was Brian Jacques saying he didn't view his books as fantasy because that was all about swords, dungeons and dragons, when his first book contained a big quest to find a sword that was found in a underground lair inhabited by a giant snake (i.e. a dragon). Even if you want to follow authors' intentions absolutely he was, by his own admission, quite confused about what he was writing.