Edit: I just learned that things have changed quite a bit since I looked into computer chess more deeply, and Stockfish does actually not use a book. So they di

Edit: I just learned that things have changed quite a bit since I looked into computer chess more deeply, and Stockfish does actually not use a book. So they did play the default config. The below is thus completely off. -- These engines are built for results, not as technical demonstrators. You are testing the engine in a scenario that it was not built to cover. Opening books are not optional add ons for these engines, they mean that the heuristics are not tuned for early game, and no work is put into optimizing the evaluation in that phase of the game. If they could have beaten Stockfish in its default configuration (using book), they wouldn't have artificially weakened it, right?