That really is an excellent point.
My feeling from several of the published games was that AlphaZero was stunning at weighing complex positional factors (some of them at least partially comprehensible to most chessplayers!) against material, and Stockfish was found wanting at getting the correct balance, with an over-materialistic tendency. However, the sample is so small it can only tell a little of the story.
The "opening book" argument doesnt have any sense. Just look at the games, Stockfish came out of the openings well in every game, close to equality in each one, it was simply crushed in the middlegames.