SSAO doesn't sample a path across the depth buffer (because that wouldn't make any sense at all for shading), or consider light direction. This is very much my recollection from many many years ago, but basically my understanding is, but I recall that it's just a random sample of depth information surrounding a given fragment, and uses the depth of those samples relative to the current fragment as a scaling factor for the ambient contribution to to the final fragment light. The end result isn't even remotely close to correct, but again, correct isn't the same as being looking good enough (or even just looking good - in games you don't have totally control of how a scene may be seen by a player. Incorrect lighting can often be far superior to correct lighting from a gameplay pov.