I think the Mistborn series takes place in the "Cosmere" but are not counted as a prequel to The Stormlight Archive. (I haven't yet read the Mistborn series, just going off of what I've heard)
Yeah, calling it (or any of the other Cosmere books) prequels is a bit of a stretch. I'm not really sure that Sanderson has even spoken on the subject of when the various series occur in relation to each other, though he may have.
That said, there are clear and distinct ties between them. Those ties are becoming more obvious as additional books are published, and what were more like Easter eggs are now becoming overt parts of the story. Don't want to say too much as the way he's building the universe of the Cosmere and the intertwining overarching story is one of the most fun things about his work.