Stockfish isn't an AI, just an expert system. It was designed from the ground up to play chess.
AlphaZero is generalized AI. It was designed to learn how to do things - like play chess. The fact that it easily beat a custom-designed expert system with only a few hours of learning time is incredible. It's a different order of complexity altogether. It may not be human intelligence, but it's a great deal closer to how humans function than how machines (like Stockfish) function.
