Brett, you are absolutely right. But my example laid the box on its side, so when you turn the box right-side-up, the top of the box becomes the side of the hull. The mast protrudes from the top of the box, which is the side of the hull. So it needs to be bent sideways after it leaves the box. But it must go some distance upwards before turning sideways, to allow the boom to clear the forbidden area.
Unfortunately, this is a case where a picture is worth a thousand words.
But we shouldn’t worry about it, since the idea is impractical anyway. As I mentioned in a later post, the large turning moment of the off-center sail would cause the boat to go in circles. But it provided a few hours entertainment.