According to Persona's damage control article[0], the subdomain had "onyx" in its name because that's the internal code name for the project, and it's named after the pokémon Onyx. No connection to Fivecast ONYX.
[0] https://withpersona.com/blog/post-incident-review-source-map...
I don't really understand why ICE would have a Persona OPenAI connection...?
Really? Sounds like they are a customer.