I feel like this is a distinction without a real difference when you're including Indiana Jones and The Dead, Mad Max and so on, all of which were essentially made as stand-alones with no specific intentions of further sequels or even exploring that subject further, and which all work as stand-alones. Literally the only difference what the author said in an interview. Functionally I would go as far as to say the Dollars trilogy is more of a consistent trilogy, on every possible level, than say, The Dead or Mad Max, and I say that really liking all of those movies.
I'm not going to argue that specific point further but it seems like if all you're going on is "the author said" like that, you're creating a rather unnecessary and hugely arguable situation. Like planting landmines in your own garden really.
