Apple won't take the risk of being blamed for AI answers being incorrect. They will attribute Google/Gemini so users know how to be mad at if it doesn't work as expected.
Apple's brand is so dominant that even if they say Siri is "powered by Google", most users will still perceive it as an Apple service. The only way that changes is if Apple consistently and prominently surfaces the Google name on Siri — which seems unlikely (but who knows when the stakes are so high).
They will do everything possible to avoid that and so re-brand is the only likely outcome.