How is screen space ambient occlusion implemented?
As Alan and trichoplax mention in the comments, the effect that ambient occlusion simulates is not the occlusion of a surface from the camera but the surface’s occlusion from its surroundings.

Think of it this way: say you have even illumination coming from every direction, so that the total incoming light at any point sums up to a value of 1. If you place a flat plane in that environment and look at one side of it, that side is going to receive 50% of that illumination, or 0.5, because the other half is blocked by the plane itself. In other words, any point on the plane’s surface can only “see” the light coming from half of the environment, so it’s half as brightly illuminated. If you fold that plane towards your viewpoint (a “valley” fold), then you decrease the incoming illumination to that side of the plane further, to some value below 0.5, because, again, each point on the plane “sees” a bit less of the light coming from the surroundings.

Screen-space ambient occlusion works more or less by looking for these “folds”—areas where the depth changes sharply, as defined by comparing the depths of neighboring pixels—and darkening them to simulate the decreased illumination from the points’ environment.