Yes, you're dead on:

  Rui Takenaka, an elite-level player who has won and lost matches against Ace, said in comments provided by Sony AI: "When it came to my serve, if I used a serve with complex spin, Ace also returned the ball with complex spin, which made it difficult for me. But when I used a simple serve - what we call a knuckle serve - Ace returned a simpler ball. That made it easier for me to attack on the third shot, and I think that was the key reason why I was able to win."

It seems like the human players might be playing in a way that tacitly overestimates their AI opponents' intelligence and underestimates their skill. AFAIK the SOTA Go AIs are still vulnerable to certain very stupid adversarial strategies that wouldn't fool an amateur (albeit they're not something you'd come up with in normal play, more like a weird cheat code). I wonder if this could get ironed out with a bit more training against humans vs. simulation.