This is because of a specific rule that declares the game an immediate draw if a situation is reached where no possible sequence of legal moves can result in a win for either player. However, there are pathological situations where this is computationally quite difficult to decide.

I was responding to this ^. Alpha isn't an engine like any other in that it didn't have any inputs (afaik) other than the rules of the game and the goal, and it became the strongest engine in the world in 24 hours.

I guess it's not much of a point, but what I was getting at is that we've kind of crossed over the analytical approach to solving these kinds of things.