Yes, I sort of suspected that that was the frame structure. I don't know however if the opening extends up the wall (possibly). I know it goes under the drywall but if it goes up behind it I don't know. I can see framing in there (vertical wood beams) and a joist (horizontal)that goes under the floor which is attached to the subfloor by a metal rod or long bolt of some kind. But you are correct in saying that the subfloor does NOT extend under the framing. In other words, the walls are not built upon the floor.
My next question is, do ballon framed houses tend to have more floor sagging than "western framed" houses? Also, I have been reading some posts about replacing/sistering floor joists and wouldn't it seem that if that is something that would need to be done eventually, wouldn't it be easier to do in a balloon framed house as the subfloor doesnt run under the wall framing and would be easier to remove as a result?