It's not that people can't stand on one foot, it's that if you stand on this wheel with only one foot, then you'll fall sideways before your other foot has a chance to hit the ground. Well, that's my assumption, anyway. Get on a (normal) bike, ride along, then stand up on it so your weight is on the pedals through your feet. Come to a stop, then try and get off it, from standing, without supporting yourself with handlebars or seat.
I've had an opportunity to try this contraption (or something remarkably similar) and this problem is academic: it's very much unlike a bike, but you'll learn how to stop and dismount without falling off, very quickly (haven't really tried whether I could stop w/o dismount, as I've played with it for a few minutes at most). It's similar to unicycles in this respect (note how in the videos, users ride back and forth a little bit, instead of coming to a complete stop).
OK fair enough, I've never tried anything like it - was just explaining why it wouldn't be quite as simple as "can you stand on one leg".
Not even being snarky.