Maybe I don't understand this thought experiment. Say you are on a plane cruising at 500 mph. The plane is applying constant thrust to counteract atmospheric drag in order to maintain steady 500 mph. At this point your horizontal speed is 500 mph and your vertical speed is zero, as the plane is maintaining level flight. When you decide to jump, you start accelerating towards the ground at roughly 9.8 m/s^2. Your horizontal speed is still close to 500 mph, but it starts decreasing due to atmospheric drag. So you are falling towards the earth, but you are also moving at a considerable horizontal speed relative to the ground due to newton's first law. The plane keeps apply thrust to maintain 500 mph. Eventually you'll land, behind the place but a considerable distance away from where you started.
I may have been imprecise: I'm talking about jumping inside the plane, not wingwalking. There's no drag inside the plane.
If you've ever taken a pee standing up on a plane, you'll notice that your pee doesn't veer rearward. You can juggle balls without losing them (you can try this in the back of a car). If you throw a ball forwards on a plane, it doesn't come back to you, and those carts that are rolled up and down the aisles aren't constantly trying to flee towards the back of the plane.
Well yes, standing inside the plane is a very important distinction. If you are standing inside and decide to jump, there is no atmospheric drag inside the plane to slow down your horizontal speed because any atmosphere in the plane is moving at the same speed you are and so the relative speed is zero.
