Robborider, I'm afraid your wonderfully named Wagtendonk has given the standard explanation of ground effect that every student QHI can draw in his sleep. His disclaimer that the real explanation for ground effect is far more complex, just indicates that he doesn't know either.
No-one doubts that the AoA in an IGE hover is reduced by the effect of the ground slowing down the downwash - what is missing is a credible expanation for how it happens without being a function of a pressure increase .
In some mysterious way, despite the concept of divergent ducts giving a reduction in speed and an increase in pressure, we must believe that simply the presence of the ground slows down the downwash and reduces induced flow.
At least the explanation that Dave Jackson posted shows how the circulation theory of lift can give an explanation without using pressure or magic - unlike some of the posts here.