The way you write it makes it sound very complicated, but in this situation, I would definitely think "we better be really careful about who we hire here in order to avoid people making the connection with the movie voice, unless we can actually get Scarlett Johansson to do the voice", and that thought process would take less than 5 seconds.
And it is not unusual at all for there to be things that everyone knows should not be written down, but either discussed only in person, or left implicit. There is usually a few slip ups though, which would come out in discovery.
> "possibility of unconscious bias when selecting among auditions"
I think "conscious but not stated to the actress" is the more likely explanation, that is not inconsistent with this reporting.
For what it's worth, if this does go to court (which I doubt), and there is discovery and depositions, and they don't find any documentation, or get any statements suggesting that this was indeed understood to be the goal, then I would be a lot more convinced.
But I think it's a giant stretch to have the base case be that nobody thought of this and they were all shocked, shocked! that people made this connection after they released it.