Evolution is not exactly a random walk, it's a lot better. But it's plausible that creating an AI more "intelligent" than humans (whatever that means) is beyond

Evolution is not exactly a random walk, it's a lot better. But it's plausible that creating an AI more "intelligent" than humans (whatever that means) is beyond our current intellectual capabilities, although I seriously doubt it. He's forgetting two other scaling modes applicable to human intelligence: accumulating knowledge and collaborating. By accumulating knowledge we can build upon the work of the generations before us and take essentially unlimited time to crack a problem. Collaboration allows not only parallel processing of problems but also interacting and exchanging inspiration towards the solution of a single problem; both which scale quite well with a lot of intelligent people. Note that there are almost trivial arguments showing some problems are beyond the means of our finite brain (viz unbounded algorithmic complexity or the halting problem).