This is actually really cool, because an upside down airfoil will still create a high pressure ridge toward its leading edge. This causes air that would ostensibly flow along the bottom (high pressure) surface to sort of reverse and end up being pushed to the upper (low pressure) surface. The separation point is further down the leading edge than would be intuitively expected. This means the top stream of air
still goes further, and faster, than the bottom stream of air.
So inverted wings still fly, just less efficiently.