Earlier quoted context omitted.
I want to add that opening books are not THAT effective in computer chess either. Yes, they are significantly more effective than in computer go (because of the smaller branching factor and greater role of tactics). However, exponential increase in game tree size is crazy, even in chess. Thus, opening books really can't take you THAT far into a chess game (10 moves). An engine with a worse book but better search/eval…
The main purpose of opening books is to beat any player who is strictly relying upon Stockfish (or LeelaZero's) output to play the game. Because these engines play very deterministically, you can beat people who just play's Stockfish #1 (or LeelaZero #1) move over and over.
In my opinion, it's really strange to describe this as an expert "beating" AlphaGo, when really it's just a technique for making AlphaGo stronger than it is without a huge pre-calculated cache of moves.