Aerodynamics of Road Vehicles (5th edition - Pages 298 - 299) has a crystal clear diagram and description of rear lift variation with squareback and fastback configurations.
The diagram:
Note how lift decreases as the shape moves from fastback to squareback ie a fastback shape has higher lift.
The description:
Note how the explanation has nothing to do with flow separation but instead to do with the airflow wrapping around that rear fastback curve.
So this is of course yet more evidence that Aerohead's theory of lift on modern car shapes is completely wrong.
(I should add that the book defines a fastback as having attached flow to the trailing edge of the car.)