My understanding is that "which means" only makes sense with the assumption that what is being studied is the laminar flow of an incompressible fluid (which was described as a fair assumption for air and a wing at subsonic speed). But thinking more about it, it's probably right that this isn't about the fact that the air above needs to travel a longer distance, which would also be true for a concave wing section, but the fact that the layers immediately above the wing need to travel the same X distance through a thinner Y section - as in a tube which becomes thinner. Which forces the fluid to go at a higher speed, and have a lower pressure.