An excellent point. And indeed, "Cindy" might be a hybrid entity, with one operator guiding an AI response tree based on her own vocal pattern, monitoring many such calls at once, and dropping in to speak directly where realtime resolution was needed -- I doubt that's on the market quite yet (particularly the "own voice" part), but it no doubt will be. Cyborg Cindy!
It's possible this Cindy was a real human failing the Turing Test due to some constraints (including pauses) imposed by robots. I don't think so -- there were other tells -- but it's possible!