They cannot disclose the identity of the person involved in order to protect their privacy. This is a very convenient cloak, leaving no way to know if they cloned Scarlet Johansson's voice or if it's from someone else.
Voices such as Shatner or Walken are at least as much about speech patterns than the voice, giving you another axis to compare voices against, so I can somewhat see those as being trademarkable. But when I hear the ChatGPT voice, it just sounds like "slightly-flirty generic female voice 03" to me.