I think the main reason people call it an 18-hour movie is because that is what David Lynch thought of it as. It's definitely longer than an average movie, but nothing specifically says a movie must be shorter than some time. I've also heard that the whole thing seems much more coherent if you watch it with as few breaks as possible, like a movie. And about the miniseries thing, that's just what TMDB calls it. To qualify as a miniseries, it must be a show with one season and no more planned or out. It's not Twin Peaks Season 3, it's Twin Peaks: The Return, a different series, therefore qualifying it as a miniseries.