My reading of the fine print (IAAL, FWIW) is that turning off Gemini Apps Activity does not affect whether human review is possible. It just means that your prompts won't be saved beyond 72 hours, unless they are reviewed by humans, in which case they can live on indefinitely in a location separate from your account.
I also asked Gemini (not Ultra) and it told me that there is no way to prevent human review.