The explanation isn't a very clear one, simply stating that the "iPad mini intelligently recognizes whether your thumb is simply resting on the display or whether you’re intentionally interacting with it." At the least, that makes it clear that the mini will ignore a finger resting on the edge of the screen. What isn't clear is what happens if that "resting thumb" stays there when another finger comes in to interact with the device, say to turn a page in a book.
It would make sense that it would act like a single input gesture. As the info states, this is a "the kind of detail you’ll notice — by not noticing it." It would be pretty noticeable if a resting thumb triggers a mult-input gesture, so we don't expect that, but we'll have to see when the mini is available next week.