This is no longer the case. Siri as of iOS 27 is a gemini-based LLM system. It has a chatbot app and everything.
I use and have used chatGPT voice while driving for awhile ... here's a very silly example https://www.instagram.com/p/DecaL9Gx6li/ . Besides that silly example im in school now and give chatGPT all my notes to create interact multiple choice questions i take while driving. Overall love hearing how other people are using chatGPT voice and or any other AI voice conversationalist tech.
I use conversational voice AI while driving and with the new Siri i constantly have to use hey siri to keep the conversation going.