The point is not that it's likely (or practically possible) that such a brain would come into being spontaneously. The point is that it is (mathematically) still more likely than some other theories about the formation of the universe (namely those that give it the order we see, low entropy, by "miraculous" happy accident), thus showing that those theories are most likely nonsense, by reductio ad absurdum. (Those other theories can't be true, otherwise we might as well find brains in space.)
