There are multiple parts to the voice performance of ChatGPT - the voice (vocal traits including baseline pronunciation) plus the dynamic manipulation of synthesized intonation/prosody for emotion/etc, plus the flirty persona (outside of vocal performance) they gave the assistant.
The fact that the baseline speaking voice of the audition tape matches baseline of ChatGPT-4o only shows that the underlying voice was (at least in part, maybe in whole) from the actress. However, the legal case is that OpenAI deliberately tried to copy SJ's "her" performance, and given her own close friends noting the similarity, they seem to have succeeded, regardless of how much of that is due to having chosen a baseline sound-alike (or not!) voice actress.