That's true. I think it works
That's true. I think it works well with the outline of the pickup. The air moves up and over the hood and follows to the windshield. At the windshield, the air then is shunted to the roof where it encounters that lip. Why is it there? It could be to break up the low pressure area at the rear of the roof. If it were not there, then the low pressure area would be rather and upset the aerodynamics. The lip not only directs the airflow but it also sets up turbulence so that the low pressure is essentially nulled. It helps the overall aerodynamic outline.