In the promo video, they say "it's small enough to fit in one hand" at time 0:49, but later at 2:29 Ives says, "you can still pick it up and easily use it with one hand."
Clearly he meant that you can pick it up with one hand, and use it with the other. Nobody thinks you can physically operate the ipad mini with a single hand.
You can probably scroll or flip pages with your thumb, so it might be comfortable to read long content with one hand (like a kindle).